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THE ANATOMY
of the
NERVOUS SYSTEM
FROM THE STANDPOINT OF DEVELOPMENT AND FUNCTION
By
STEPHEN WALTER RANSON, M. D., Ph. D.
Professor of Anatomy in Northwestern University Medical School, Chicago
WITH 260 ILLUSTRATIONS
SOME OF THEM IN COLORS
PHILADELPHIA AND LONDON
W. B. SAUNDERS COMPANY
1921
Copyright, 1920, by W. B. Saunders Company
Reprinted April, 1921
PRINTED IN AMERICA
PRESS OF
W. B. SAUNDERS COMPANY
PHILADELPHIA
PREFACE
IN the pages which follow the anatomy of the nervous system has been pre-
sented from the dynamic rather than the static point of view; that is to say,
emphasis has been laid on the developmental and functional significance of struc-
ture. The student is led at the very beginning of his neurologic studies to think
of the nervous system in its relation to the rest of the living organism. Struc-
tural details, which when considered by themselves are dull and tiresome, become
interesting when their functional significance is made obvious. This method of
presentation makes more easy the correlation of the various neurologic courses
in the medical curriculum. For physiologic and clinical neurology a knowledge
of conduction pathways and functional localization is essential, and this informa-
tion can best be acquired in connection with the course in anatomic neurology.
In selecting the material to be included in this book the needs of the medical
student have been kept constantly in mind, and emphasis has been placed on
those phases of the subject which the student is most likely to find of value to
him in his subsequent work.
In many laboratories the head of the shark and the brain of the sheep have
been used to supplement human material. The book has been so arranged as to
facilitate such comparative studies without making it any the less well adapted
to courses where only human material is used.
During the past twenty years very considerable additions have been made to
the science of neurology, and the more important of these have been included in
the text. While a detailed presentation of the evidence concerning new or dis-
puted points would be out of place in a book of this kind, whenever the state-
ments made here differ from those found in other texts the authority has always
been cited, the author's name and the date of his contribution being given in
parentheses. A full list of these references to the literature has been included in
a Bibliography at the end of the volume.
The terminology adopted is that of the B. N. A., which has been used, for
the most part, in its English form. But in the case of the fiber tracts the Basle
12 PREFACE
terms are often misleading, and wherever this is the case, other names have been
substituted.
An outline for a laboratory course in neuro-anatomy has been included, and
this has been so arranged as to be easily adapted by the instructor to his par-
ticular needs.
Free use has been made of material gathered and arranged by others in the
various handbooks, texts, and atlases that deal with the nervous system. The
classification of the afferent paths and centers adopted here is based on the
work of Sherrington. The terms which he introduced and which are now coming
into general use have been employed. In the analysis of the cranial nerves the
American conception of nerve components, so ably presented by Herrick, has
been utilized.
Illustrations have been borrowed from many sources, in each case duly
accredited, and our indebtedness for permission to use them is gladly acknowl-
edged. The majority of the figures have been made from drawings prepared for
this purpose by Miss M. E. Bakehouse. The large number of illustrations and
the excellent manner in which they have been reproduced is to be credited to the
generous policy of the publishers, W. B. Saunders Co. My thanks are due to
Dr. Olaf Larsell for reading the manuscript and for many valuable suggestions,
and to Mr. Michael Mason for assistance in reading the proof.
S. W. RANSON.
CHICAGO, ILL.
CONTENTS
CHAPTER I PAGE
ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 17
The Diffuse Nervous System of Ccelenterates 19
The Central Nervous System 20
CHAPTER II
THE NEURAL TUBE AND ITS DERIVATIVES 24
The Brain of the Dogfish 26
Development of the Neural Tube in the Human Embryo 31
CHAPTER III
HlSTOGENESIS OF THE NERVOUS SYSTEM 37
Development of the Neuron 37
Development of the Spinal Nerves 40
Differentiation of the Spinal Cord 42
CHAPTER IV
NEURONS AND NEURON-CHAINS 43
Form and Structure of Neurons 43
Interrelation of Neurons 49
The Neuron as a Trophic Unit 51
The Neuron Concept 52
Neuron Chains 53
CHAPTER V
THE SPINAL NERVES 56
Metamerism 58
Functional Classification of Nerve-fibers : 60
The Spinal Ganglia 62
Somatic Sensory Fibers and Nerve Endings 66
CHAPTER VI
THE SPINAL CORD 73
External Form and Topography 73
The Spinal Cord in Section 78
Microscopic Anatomy 85
The Spinal Reflex Mechanism 91
CHAPTER VII
FIBER TRACTS OF THE SPINAL CORD 95
Intramedullary Course of the Dorsal Root Fibers 95
Afferent Paths in the Spinal Cord 98
Ascending and Descending Degeneration in the Spinal Cord 105
Long Descending Tracts of the Spinal Cord 108
CHAPTER VIII
GENERAL TOPOGRAPHY OF THE BRAIN 113
Anatomy of the Medulla Oblongata 118
Anatomy of the Pons 123
The Fourth Ventricle 125
The Mesencephalon 129
13
14 CONTENTS
CHAPTER IX PAGE
THE STRUCTURE OF THE MEDULLA OBLONGATA 132
The Rearrangement Within the Medulla Oblongata of the Structures Continued Upward
from the Spinal Cord 133
Decussation of the Pyramids 136
Nucleus Gracilis, Nucleus Cuneatus, and Medial Lemniscus 137
Olivary Nuclei 141
Restiform Body 143
Formatio Reticularis 144
CHAPTER X
INTERNAL STRUCTURE OF THE PONS 147
Basilar Part of the POPS 147
Dorsal Part of the Pons 149
CHAPTER XI
INTERNAL STRUCTURE OF THE MESENCEPHALON 158
Tegmentum 158
Basis Pedunculi 164
Corpora Quadrigemina 165
CHAPTER XII
THE CRANIAL NERVES AND THEIR NUCLEI 168
Somatic Efferent Column of Nuclei 170
Special Visceral Efferent Column of Nuclei 174
General Visceral Efferent Column of Nuclei 177
Visceral Afferent Column 180
General Somatic Afferent Nuclei 182
Special Somatic Afferent Nuclei 185
Summary of the Origin and Composition of the Cranial Nerves 190
CHAPTER XIII
THE CEREBELLUM 195
Development 195
Anatomy 196
Morphology 199
Nuclei of the Cerebellum 203
Cerebellar Peduncles 204
Histology of the Cerebellar Cortex 206
Efferent Cerebellar Tracts 211
CHAPTER XIV
THE DlENCEPHALON AND OPTIC NERVE 213
Thalamus 213
Epithalamus and Metathalamus 220
Hypothalamus 222
Third Ventricle 223
Visual Apparatus 225
CHAPTER XV
EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 229
Development of the Cerebral Hemispheres 229
The Dorsolateral Surface 232
The Medial and Basal Surfaces . . . 238
CONTENTS 15
CHAPTER XVI PAGE
INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 243
Corpus Callosum 243
Lateral Ventricles 246
Basal Ganglia of the Telencephalon : 252
Internal Capsule 257
Connections of the Corpus Striatum and Thalamus 262
CHAPTER XVII
THE RHINENCEPHALON 265
Parts Seen on the Basal Surface of the Brain 265
Hippocampus 269
Fornix 270
Anterior Commissure 273
Structure and Connections of the Several Parts of the Rhinencephalon 274
Olfactory Pathways 280
CHAPTER XVIII
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 283
Structure of the Cerebral Cortex 283
Cortical Areas 287
Localization of Cortical Functions 290
The Medullary Center of the Cerebral Hemisphere 296
CHAPTER XIX
THE GREAT AFFERENT SYSTEMS 302
Exteroceptive Pathways to the Cerebral Cortex 302
Spinal Path for Touch and Pressure 303
Spinal Path for Pain and Temperature Sensations 306
Secondary Trigeminal Paths 307
Neural Mechanism for Hearing 309
Neural Mechanism for Sight 310
Proprioceptive Pathways 311
Spinal Proprioceptive Paths (Muscle Sense) 311
Cerebellar Connections of Vestibular Nerve 314
CHAPTER XX
EFFERENT PATHS AND REFLEX ARCS 316
The Great Motor Path 317
The Cortico-ponto-cerebellar Path 325
The Cerebello-rubro-spinal Path 326
Important Reflex Arcs 327
CHAPTER XXI
THE SYMPATHETIC NERVOUS SYSTEM 334
Fundamental Facts Concerning Visceral Innervation 335
Structure of the Sympathetic Ganglia 341
Composition of Sympathetic Nerves and Plexuses 345
Architecture of the Sympathetic Nervous System 346
Important Conduction Paths Belonging to the Autonomic Nervous System 352
A LABORATORY OUTLINE OF NEURO-ANATOMY 355
BIBLIOGRAPHY . . 375
INDEX . . 383
THE ANATOMY OF THE NERVOUS SYSTEM
FROM THE STANDPOINT OF DEVELOP-
MENT AND FUNCTION
CHAPTER I
THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM
IRRITABILITY and conductivity, which, as every biological student knows,
are two of the fundamental properties of protoplasm, reach their maximum
development in the highly differentiated tissue of the nervous system. Indeed,
it is in response to the need for increased sensitiveness to stimuli and for better
transmission of the impulses aroused by them that the nervous system has
developed and been perfected in the long process of evolution which has cul-
minated in man.
When an ameba is touched with a pointed glass rod it moves away from
the source of stimulation. Changes are initiated in the superficial protoplasm
which are transmitted through the unicellular organism, resulting in a flowing
out of pseudopodia on the opposite side. Through a continuation of this stream-
ing motion the entire organism moves forward. Thus the relatively undif-
ferentiated living substance of which it is composed receives the stimulus,
transmits the resulting disturbance, and carries out the appropriate response.
When in the place of unicellular organisms we study simple metazoa, the
sea-anemones for example, we find that considerable differentiation has occurred
among the component cells. A cuticle has formed, designed to protect the
subjacent parts from the action of the surrounding objects, while other cells
have differentiated in the direction of contractile elements or muscle cells.
Because the general body surface has been adapted to cope with the environ-
ment it becomes necessary to have certain cells at the surface which are sensi-
tive to environmental changes. These sensory elements are able to transmit
the waves of activation developed in them directly to the subjacent muscle
cells. But in higher animals, because of the large size of the body and the
2 17
i8
THE NERVOUS SYSTEM
complicated reactions required, long lines of communication have been estab-
lished between peripheral sense organs and muscle-fibers in widely separated
parts of the body.
The sensory elements and the lines of communication constitute the nervous
system and, together with the musculature, the neuromuscular mechanism.
It is well to keep in mind the fact that the nervous system was developed for the
purpose of enabling the musculature to react to changes in the environment of
the organism. But in all higher animals the nervous system responds not only
to stimuli from without but also to stimuli from within the body, and helps to
Itl
Fig. 1. Stages in the differentiation of the neuromuscular mechanism: A to C, Hypothetic
early stages: A, epithelial stage; B, muscle cell at the stage of the sponge; C, partially differen-
tiated nerve-cell in proximity to fully differentiated muscle-cell; D, nerve- and muscle-cell of
coelenterate stage; E, a type of receptor-effector system found in many parts of sea-anemones, in-
cluding not only receptors, r, with their nerve-nets, and of muscle cells, w, but also of ganglion
cells, g, in the nerve-net; F, section at right angles to the sphincter of the bell of a jellyfish (Rhizos-
toma): e, epithelium of the subumbrellar surface; n, nervous layer; w, muscle layer. (Parker.)
bring about an internal adjustment of part with part. Here again it acts as a
sensitive mechanism for receiving stimuli and conducting them to the appro-
priate organs of response. These organs through which the nervous system
produces its effects are known as effectors. While muscles and glands are by
far the most important effectors, we must also include certain pigmented cells
(or chromatophores) and electric and phosporescent organs under this heading.
Except for the reactions produced through such effectors the nervous system
would be meaningless.
We can best understand the significance of the nervous system if we trace
its early history. This, as it has been interpreted by Parker (1919), makes an
THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM
interesting story. According to this author contractile tissue develops before
any trace of the nervous system appears. In sponges, which are devoid of
nervous elements, the oscula open and close in response to appropriate stimuli.
These movements are brought about by a contractile tissue not unlike smooth
muscle. The active element or effector is thus the first to make its appearance,
and at this stage is brought into action by direct stimulation. Next in the order
of development is the sensory cell, derived from the epithelium in the neigh-
borhood of an effector, and specially differentiated to receive stimuli and trans-
mit them to the underlying muscle (Fig. 1, D). This stage of development is
reached by such ccelenterates as the sea-anemones. The advantage which
these forms derive from the specialized sensory cells or receptors is seen in the
character of their responses, which are more rapid than those of sponges. Such
Cerebral ganglion-
Esophageai connective ---
Pharynx
Ventral nerve cord --^
.... Cerebral ganglion
-- Pharynx
Esophageai connective
- Ventral nerve cord
A B
Fig. 2. Anterior portion of the nervous system of the earthworm: A, Lateral view; B, dorsal view.
a sensory cell may be compared to a percussion cap through which a charge of
powder is ignited.
But ccelenterates usually present a more complex arrangement of receptor
and effector elements than that indicated in Fig. 1, D. Fine branches from the
sensory cells anastomose with each other and form a nervous net within which
are scattered nerve-cells. Such a nerve net is seen in many parts of sea-ane-
mones (Fig. 1, E) and is well developed in the jellyfish (Fig. 1, F). It seems
capable of conveying nerve impulses coming from the sensory cells in all direc-
tions through the bell-shaped body of the jellyfish and to muscle-fibers far dis-
tant from the receptors involved. The conduction of nerve impulses from
receptors to effectors seems to occur diffusely through the net not in stated
directions nor along fixed paths. In this respect the diffuse nervous system of
the ccelenterates is in contrast with the more centralized system in the worms.
20
THE NERVOUS SYSTEM
The sensory cells are not so directly connected with muscle-fibers hi the
worms as in the sea-anemones, for between receptor and effector there is here
interposed a central nemous system. This system, as it appears in the earth-
worm, is illustrated in Fig. 2. It consists of a cerebral ganglion dorsal to the
buccal cavity and a row of ventrally placed ganglia bound together by a ventral
nerve cord. The most anterior of the ventral series of ganglia is connected to
the dorsal one by nerve strands on either side of the esophagus. The ganglia
of the ventral cord are placed so that one occurs in each body segment, and
from each three pairs of nerves run to the skin and muscles of that segment.
The arrangement of the constituent elements can best be studied in transverse
sections (Fig. 3). The sensory cells are located in the skin, and from each of
them a fiber runs along one of the nerves into the ganglion, within which it
branches, helping to form a network known as the neuropil. Within each
Fig. 3. Transverse section of the ventral chain and surrounding structures of an earthworm:
cm, Circular muscles; ep, epidermis; Im, longitudinal muscles; me, motor cell-body; mf, motor
nerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion. (Parker.)
ganglion are found large nerve-cells from which fibers run through the nerves
to the segmental musculature. Here we have the necessary parts for the sim-
plest reflex arc. Stimulation of the sensory cell causes nerve impulses to travel
through its fiber to the neuropil, thence to a motor cell, and finally along a proc-
ess of the latter to the muscle. In other words, we have a receptor, conductor,
center, another conductor, and finally an effector; and all this is for the purpose
of bringing the muscle-fiber under the influence of such environmental changes
as are able to stimulate the sensitive receptor.
In addition to the primary sensory and motor elements just enumerated the
ganglia contain nerve-cells the fibers of which run from one ganglion to another
and serve to associate these in co-ordinated activity. These internuncial ele-
ments serve to establish functional connections among the different parts of
the ganglionated nerve cord that constitutes the central nervous apparatus;
THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 21
and they lie entirely within this central organ. The slow waves of contraction
that pass from head to tail as the worm creeps forward may be advanced from
segment to segment by such internuncial or association elements.
The nervous system of the earthworm differs from that of the ccelenterate
in many ways, but the fundamental difference is one of centralization. In the
former the greater part of it has separated from the skin and become con-
centrated in a series of interconnected ganglia which serve as a central nervvts
system. These ganglia receive nerve-fibers, coming from the sense organs, and
give off others, going to the muscles; and the fibers are brought together and
grouped into nerves for convenience of passage. The neuropil within a ganglion
offers a variety of pathways to each incoming impulse which may accordingly
find its \vay out along one or more of several motor fibers. The spreading of
nerve impulses through the chain of ganglia is facilitated by the presence of the
association fibers already mentioned. Nevertheless, conduction is not diffuse
as in the nerve net of the medusa, but occurs along definite and more or less
restricted lines. This is well illustrated by the experiment cited by Parker:
"If an earthworm that is creeping forward over a smooth surface is suddenly
cut in two near the middle, the anterior portion will move onward without much
disturbance, whereas the posterior part will wriggle as though in convulsions.
This reaction, which can be repeatedly obtained on even fragments of worms,
shows that a single cut involves a stimulation which in a posterior direction
gives rise to a wholly different form of response to what it does anteriorly; in
other words, transmission in the nerve cord of the worm is specialized as com-
pared with transmission in the nervous net of the ccelenterate." In the gan-
glionated cord of the earthworm, as here described, we find many of the features
characteristic of the central nervous system of higher forms.
The vertebrate nervous system has much in common with that of the earth-
worm. The central nervous system of the annelid is split off from the ectoderm
by a process of delamination, as will be seen by comparing the ventral nervous
cord of the marine worm, Sigalion, with that of the earthworm (Figs. 3, 4).
Through a comparable process of infolding of the ectoderm to form a neural
tube there is developed the central nervous system of the vertebrate (Fig. 6).
The dorsal position of the neural tube in vertebrates as compared with the
ventral position of the solid nerve cord of the annelid offers some difficulty and
has led to ingenious theories in explanation of their phylogenetic relationship,
theories which we need not consider here (Gaskell, 1908). In primitive chor-
dates, such as the amphioxus, we already have a simple, dorsally placed, neural
22
THE NERVOUS SYSTEM
tube associated with segmental nerves. In true vertebrates the anterior end of
the neural tube becomes irregularly enlarged to form the brain, while the pos-
terior end remains less highly but more uniformly developed and forms the
spinal cord.
The primary motor nerve-cells of vertebrates resemble very closely those of
invertebrates in being located within the central nervous system and in send-
ing motor nerve-fibers to the muscles (Fig. 31). The primary sensory cells lie
outside the central system, as in invertebrates. Those for smell are located in
the olfactory epithelium. But all others have migrated centrally along the
sensory fibers, and now send one process toward the periphery and another into
bra
Fig. 4. Transverse section of the ventral nervous cord of Sigalion: bm, Basement mem-
brane; c, cuticula; e, epidermis; gc, ganglion-cells; n, nerve-fibers and neuropil; s, space occupied
by vacuolated supporting tissue. (Parker, Hatschek.)
the central system. The relative positions of these cells in the annelid, mollusc,
and vertebrate are illustrated in Fig. 5. In the latter the sensory cells are aggre-
gated into masses known as the cerebrospinal ganglia, which are associated
with peripheral nerves and are usually placed near the point of origin of these
nerves from the brain or spinal cord. A comparison of Figs. 3 and 31 will show
a striking similarity between the simple reflex arc in the earthworm and in man.
If space permitted we might trace the development of the central nervous sys-
tem in some detail, but perhaps enough has been given to suggest that the
nervous system of man represents the culmination of a long process of evolu-
tion which began with a simple sensory mechanism like that of the sea-anemones.
We shall be concerned with a study of the vertebrate nervous system, almost
THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM
2 3
exclusively with that of the mammal, and more particularly with that of man.
In man we are so accustomed to think of the nervous system as the organ and
agent of the mind that its true physiologic position is often forgotten. In this
introductory chapter we have attempted to show that the primary function of
the nervous system is to receive stimuli arising from changes in the environment
or within the organism, and to transmit these to effectors which bring about
the adjustments necessary for life. Biologically speaking, the nervous system
is not to be regarded as an intelligence bureau, which gathers information for
CO-
1
FZ
1
>
F
h-
X'
Fig. 5. Peripheral sensory neurons of various animals: A, Oligochaetic worms (Lumbricus);
B, polychaetic worms (Nereis); C, molluscs (Limax); D, vertebrates. The figure illustrates the
gradual change in the position of the sensory cells in the phylogenetic series: e, Epithelial cells of
sensory surface; c, cuticula; sz, cell-body of peripheral sensory neuron; rm, rete Malpighii of epi-
dermis; sn, axon; co, central nervous system. (Barker, Retzius.)
a sovereign mind, enthroned within the brain, nor yet as a chief executive officer
to carry out that sovereign's decrees. Sensory impulses from many sources
reach the brain, where they pass back and forth through a multitude of asso-
ciation paths, augmenting or inhibiting each other before they finally break
through into motor paths. Previous experience of the individual, having left
its trace in the organization of the central nervous system, alters the character
of the present reactions. It is in connection with the neural activity involved
in these complex associational processes that consciousness appears shall I
say as a by-product? at least as a parallel phenomenon.
CHAPTER II
THE NEURAL TUBE AND ITS DERIVATIVES
Infolding of the Neural Tube. The vertebrate nervous system develops
from a thickened plate of ectoderm along the middorsal line of the embryo.
By the infolding of this neural plate there is formed the neural groove, which
becomes transformed into the neural tube (Fig. 6). The neural tube detaches
itself from the superficial ectoderm and gives rise through a thickening of its
walls to the brain and spinal cord. The latter is formed by a process of uniform
Neural groove Neural plate
Neural groove Neural plate
Ectoderm Neural groove
Neural tube
Neural tube
D
Neural cavity
Fig. 6. Development of the neural tube in human embryos (Prentiss-Arey): A, An early embryo
(Keibel) ; B, at 2 mm. (Graf Spec) ; C, at 2 mm. (Mall) ; D, at 2.7 mm. (Kollmann).
thickening in the walls of the caudal portion of the tube. The derivatives of
the rostral part are well illustrated in the accompanying diagram (Fig. 7).
Brain Vesicles. At an early stage in the development of any vertebrate
embryo the rostral portion of the neural tube is distinguished from the caudal
part by the more rapid development of the former, its walls bulging outward
to form three bulb-like swellings or vesicles, which together represent the brain,
and are named from before backward, the prosencephalon, mesencephalon, and
24
THE NEURAL TUBE AND ITS DERIVATIVES 25
rhombencephalon (Fig. 7). The more rostral vesicle becomes subdivided by a
constriction into the telencephalon and diencephalon (Fig. 7, B, C). The rhom-
bencephalon is less sharply subdivided into a rostral part, which includes the
cerebellum, and is known as the metencephalon, and a more caudal portion, the
myelencephalon. The optic nerves and retinae, not illustrated in the figure,
develop as paired evaginations from the prosencephalon.
The Cerebral Hemispheres. The telencephalon includes a thickened portion
of the ventrolateral wall loosely designated as the corpus striatum or, since there
Fig. 7. Diagrams illustrating the development of the vertebrate brain: A, First stage, side
view, the cavity indicated by dotted line; B, second stage; C, third stage, side view of a brain with-
out cerebral hemispheres; D, the same in sagittal section; E, fourth stage, side view of a brain with
cerebral hemispheres; F, the same in sagittal section; G, dorsal view of the same with the cavities
exposed on the right side. Rhin., rhinocoele; Lot. Vent., lateral ventricle; Int. For., interventricu-
lar foramen; Vent. Ill, third ventricle; Vent. IV, fourth ventricle. /, Prosencephalon; / a, Telen-
cephalon; I a-r, Rhinencephalon ; I a-p, Pallium; / a-lt, Lamina terminalis; / a-ch, Cerebral
hemisphere; i a-cs, Corpus striatum; i b, Diencephalon; / b-t, Thalamus. 2, Mesencephalon ; 2c,
Optic lobes; 2 d, Crura cerebri. j, Rhombencephalon; j a, Metencephalon; 3 a-c, Cerebellum;
3 b, Myelencephalon.
is one of these on either side, the corpora striata (Fig. 7, D). Another part of
the wall is relatively thin and is known as the pallium, while the part directly
associated with the olfactory nerve belongs to the rhinencephalon. The most
important factor in the evolution of the vertebrate brain is the progressive evag-
ination of the lateral walls of the telencephalon to form paired masses, the
cerebral hemispheres. In primitive forms like the cyclostomes only a part of the
rhinencephalon has been evaginated, and in them the hemisphere consists only
of an olfactory bulb and olfactory lobe. This stage of development is roughly
2 6 THE NERVOUS SYSTEM
indicated in Fig. 7, C, D. In the selachians, as illustrated in Figs. 8, 9, 10,
and 11, the evagination has progressed further than in cyclostomes. Still further
progress in this direction has been made by the. amphibians, the cerebral hemi-
spheres of which have reached about the stage of development indicated in Fig.
7, E, F, G. Here the entire lateral wall, including the pallium and corpus
striatum, has been evaginated in the formation of the cerebral hemisphere.
The Brain Ventricles. The portions of the original cavity of the neural tube
which are contained within the evaginated cerebral hemispheres are known as
the lateral ventricles (Fig. 7, G). These paired ventricles communicate with the
median prosencephalic cavity by openings known as the interventricular foram-
ina. This median cavity, called the third ventricle, represents for the most
part the cavity of the diencephalon, but its rostral part, bounded by the lamina
terminalis, belongs to the telencephalon. It will be seen by a study of the
accompanying diagrams that this lamina also belongs to the telencephalon and
represents in a certain sense the rostral end of the brain. Its position should
be carefully noted in each of the diagrams. The cavity of the rhombencephalon
is known as the fourth ventricle and that of the mesencephalon as the cerebral
aqueduct. The latter connects the third and fourth ventricles. It will help us
to understand the morphology of the vertebrate brain if we now consider the
shape and arrangement of the various parts of a simple brain like that of the
dogfish.
THE BRAIN OF THE DOGFISH SQUALUS ACANTHIAS
The telencephalon of the selachian brain is evaginated to form a pair of
laterally placed masses, the cerebral hemispheres, and in this respect is at a stage
of development not far removed from that represented in diagrams E, F, and G
of Fig. 7. The long axis of the brain is almost straight; and this freedom from
ventrodorsal curvatures makes it especially easy to recognize the various funda-
mental divisions already enumerated and to understand their relationship.
The medulla oblongata, which together with the cerebellum forms the rhom-
bencephalon, is continuous at the caudal extremity with the cylindric spinal
cord, and within it the central canal of the spinal cord opens out into the fourth
ventricle (Fig. 8). The medulla, which has somewhat the shape of a trun-
cated cone, is considerably larger than the cord, but decreases in size as it is
traced backward toward their point of junction. In the mammal a conspicuous
transverse bundle of fibers, associated with the cerebellum, is found on the
ventral and lateral aspects of that part of the medulla which belongs to the
metencephalon and is known as the pons. But in the fish it is customary to
THE NEURAL TUBE AND ITS DERIVATIVES
consider the medulla oblongata as extending from the spinal cord to the mesen-
cephalon. It forms the ventral and lateral walls of the fourth ventricle; and
when the roof of this cavity has been removed these walls are seen to surround
a long and rather broad depression the fossa rhomboidea or floor of the fourth
ventricle which tapers caudally like the point of a pen (Fig. 9).
The cerebellum forms an elongated mass the rostral end of which overhangs
the optic lobes, while the caudal extremity projects over the medulla oblongata
/'Nasal capsule
---O'factory bulb
/ -Nervus terminalis
..Olfactory tract
/ Olfactory nerve N. I
' / Rhinoccele
, Lateral ventricle
! '
Cerebral hemisphere.
Interventricular for
Epiphysis
-Optic nerve N. II
Thalamus-
Optic lobes-
-Trochlear nerve N. Ill
-Cerebellum
Lobus linecR lateralis
Facial nerve N. VIL
Acoustic nerve N. VIII.
Tuberculum acusticunt
Medulla oblongata-
Glossopharyngeal nerve N. IX
Medial longitudinal fasc
Visceral lobe.__
-Vagtts nerve N. X.
-Spinal cord.-
&? Telencephalon
-- - - -;- Third ventricle
) Diencephalon
<
---- '-Mesoccele
> Mesencephalon
f Metencephalon
-j- Cerebellum
(caudal part)
i^ -I- Rhomboid fossa
V Myelencephalon
Fig. 8. The brain of the dogfish,
Squalus acanthias, dorsal view.
Fig. 9. The brain of the dogfish,
Squalus acanthias, with the ventricles
opened, dorsal view.
(Fig. 8). Its dorsal surface is grooved by a pair of sulci arranged in the form
of a cross. It contains a cavity, a part of the original rhombencephalic vesicle,
which communicates with the fourth ventricle proper through a rather wide
opening (Fig. 11). Behind the cerebellum the fourth ventricle possesses a thin
membranous roof which was torn away in the preparation from which Fig. 8
was drawn.
28
THE NERVOUS SYSTEM
Mesencephalon. The optic lobes on the dorsal aspect of the mesencephalon
are a pair of rounded masses separated by a median sagittal sulcus. They
represent the bulging roof of the mesencephalic cavity and are accordingly
Cerebellum. ...
Optic lobe
Thalamus
Cerebral hemisphere
Olfactory bulb
v
Vagus nerve N. X /
Glossopharyngeal nerve N. IX '
Acoustic nerve N. VIII /
Abducens nerve N. VI
Olfactory tract
> ! Optic nerve N. II
\ Inferior lobe
Oculomotor nerve N. Ill
Saccus vasculosus
Trigeminal and facial nerves Nn. V, VII TroMear nene N _ IV
Fig. 10. The brain of the dogfish, Squalus acanthias, lateral view.
spoken of as the tectum mesencephali. Within this roof end the fibers which
come from the retinae through the optic nerves. The floor of the cavity is formed
by the ventral part of the mesencephalon. This appears like a direct continua-
tion of the medulla oblongata, and in the mammal bears the designation crura
Paraphysis
Cerebral hemisphere j |
Olfactory tract
Olfactory bulb
Optic lobe
Epiphysis . Mesoccde
Cerebellum
Metacode
Tuberculum acusticum
. Tela chorioidea
Fourth ventricle
Visceral lobe
Telencephalon ( !
Preoptic recess
Velum transversum
\ Metencephalon Myelencephalon
Saccus vasculosus
Mesencephalon
Optic chiasma Third ventric i e
Fig. 11. The brain of the dogfish, Squalus acanthias, medial sagittal section.
cerebri. Emerging from the roof of the mesencephalon between the cerebellum
and optic lobe is the fourth or trochlear nerve, and from the ventral aspect of
this division of the brain arises the third or oculomotor nerve.
The Diencephalon. The thin roof of the diencephalon, which can easily
THE NEURAL TUBE AND ITS DERIVATIVES
2 9
be torn away so as to expose the third ventricle (Figs. 8, 9), is attached by its
caudal margin to a ridge containing a pair of knob-like thickenings, the habe-
nular nuclei and a commissure connecting the two (Fig. 11). From a point
just caudal to the middle of this commissure there projects forward over the
membranous roof of the ventricle a slender tube, the epiphysis cerebri or pineal
body, which comes in contact with the roof of the skull and ends in a slightly
dilated extremity. The epiphysis and habenular nuclei belong to the epithala-
mus. The thalamus forms the thick lateral wall of the third ventricle and is
traversed by the optic tracts on their way to the optic lobes. The hypothalamus
Nasal sac
Epiphysis
Superior oblique
Trochlear nerve
Medial rectus
Superior rectus
Lateral rectus
Vestibule
Spiracle-
Semicircular canal
Glossopharyngeal nerve
Vagus -
Branchial cleft i
Superficial ophthalmic V, VII
Olfactory capsule
Inferior oblique
Maxillary V
Mandibular V
Palatine VII
Spiracle
Hyomandibular VII
Glossopharyngeal
i. Branchial cleft
Vagus
Spinal cord Lateral line branch of vagus
Fig. 12. Dissection of the brain and cranial nerves of the dogfish, Scyllium catulus. The
eye is shown on the left side, but has been removed on the right. (Marshall and Hurst, Parker
and Haswell.)
is relatively large in the shark and presents, in addition to a pair of laterally
placed oval masses, or inferior lobes, a thin walled vascular outgrowth, the saccus
vasculosus. Closely related to the ventral aspect of the hypothalamus is a gland-
ular mass, derived by a process of evagination from the oral epithelium, and
known as the hypophysis. For a picture of this structure in the adult dogfish
reference should be made to a paper on the subject by Baumgartner (1915).
On the ventral surface of the hypothalamus the optic nerves meet and cross in
the optic chiasma.
The telencephalon includes all of the brain in front of the velum transfer sum,
3
THE NERVOUS SYSTEM
a transverse fold projecting into the third ventricle from the membranous roof
(Fig. 11), and consists of a median unpaired portion, and of the two cerebral
hemispheres with their olfactory bulbs. The hemispheres are the evaginated
portions of the telencephalon and are partially separated from each other by a
Olfactory bulb
Olfactory nerve
(n.I)
Somatic area
r. ophthal. superfic. V
r. ophthal. superfic. VII
n. terminalis
r. ophthal. profundus V
Optic nerve (n. II)
r. maxillaris V
r. mandib. V
Supra-orbital trunk
Infra-orbital trunk
Ganglion V
r. palatinus VII
Gang, geniculi VII
Gang, later. VII
r. prespirac. VII
Spiracle
r. hyomandib. VII
n. IX
n. X
r. lateralis X
r. branchialis X
r. intestinalis X
Fig. 13. Diagram of the brain and sensory nerves of the smooth dogfish, Mustelus canis,
from above. Natural size. The Roman numerals refer to the cranial nerves The olfactory
part of the brain is dotted, the visual centers are shaded with oblique cross-hatching, the acoustico-
lateral centers with horizontal lines, the visceral sensory area with vertical lines, and the general
cutaneous area is left unshaded. On the right side the lateral line nerves are drawn in black, the
other nerves are unshaded. (From Herrick's Introduction to Neurology.)
median sagittal fissure, which has been to a large extent obliterated by the
fusion of their median walls. The shape of the lateral ventricle and the position
of the interoentricular foramina are shown in Fig. 9. From the lateral side of
the rostral end of the hemisphere there projects forward the long and slender
olfactory tract with a terminal enlargement, the olfactory bulb. This lies in
THE NEURAL TUBE AND ITS DERIVATIVES 3 1
contact with the nasal sac to which it gives off a number of fine nerve bundles,
which together constitute the olfactory or first cranial nerve. At the rostral end
of the brain an additional nerve makes its exit from the hemisphere. It is
known as the nervus terminalis and can be followed forward over the olfactory
tract and bulb to the nasal sac (Fig. 8).
The roof of the selachian forebrain presents a number ^structures of great morphologic
interest, two of which have already been mentioned, namely, the epiphysis and velum
transversum. The former is an outpocketing of the roof of the diencephalon; the latter
is an infolding and marks the line of separation between the two divisions of the prosenceph-
alon. Rostral to the velum the roof of the telencephalon is evaginated to form a thin-walled
sac, the paraphysis. The velum and paraphysis are readily identified in the mammalian
embryo, but become obscured in the course of later development. The morphology of this
region has recently been studied in great detail by a number of American investigators:
Minot (1901), Johnston (1909), Terry (1910), Warren (1911, 1917), and Bailey (1916).
A good idea of the shape and connections of the various brain ventricles and
of the relation of the various parts of the brain to each other can be obtained
from a study of Figs. 9 and 11. In Fig. 13 there is indicated the location of the
principal sensory areas of the brain of the smooth dogfish, and the relation of
these areas to the corresponding peripheral nerves is apparent. The lateral
line components of the seventh and tenth cranial nerves are indicated in black.
DEVELOPMENT OF THE NEURAL TUBE IN THE HUMAN EMBRYO
In its embryonic development the nervous system of man presents some-
thing like a synopsis of the early chapters of its phyletic history. The neural
groove is the most conspicuous part of an embryo of 2.4 mm. (Fig. 14). Near
the middle of the body it has closed to form the neural tube, and from this
region the closure proceeds in both directions. The last points to close are
situated at either end and are known as the neuropores. The rostral end of the
groove shows enlargements which upon clpsure will form the brain vesicles.
The longer portion, caudal to these enlargements, represents the future spinal
cord. Except that it is flexed on itself, the brain of the human embryo of Jive
weeks (Fig. 15) shows a marked resemblance to the diagram of a vertebrate
brain without cerebral hemispheres (Fig. 7, C, D). The prosencephalic vesicle
is divided by a constriction into the telencephalon and diencephalon with freely
intercommunicating cavities. The mesencephalon is well denned and presents
a sharp bend, the cephalic flexure. The rhombencephalon shows signs of sepa-
ration into the metencephalon and myelencephalon and is slightly bent dorsally
at the pontine flexure. Another curvature which develops at the junction of
THE NERVOUS SYSTEM
the brain and spinal cord is known as the cervical flexure (Fig. 16). From
the walls of the prosencephalon there develop outpocketings on either side,
which form the optic cups and which are connected to the brain by the optic
stalks. From the cup develops the retina and through the stalk grow the
fibers of the optic nerve. These structures are, therefore, genetically parts of
the brain.
The Telencephalon of the Human Embryo. By the time the embryo has
reached a length of 13 mm. the brain has passed into the stage represented by
Mesencephalon
Rhombenccphalon
Myelencephalon
Amnion (cut)
Mesodermal segment 14
Open neural groove
Body stalk
Fig. 14. Human embryo of 2.4 mm. showing the neural tube partially closed. (Kollmann.)
diagrams E, F, G of Fig. 7. The lateral wall of the telencephalon, with the
corpus striatum and olfactory brain or rhinencephalon, has been evaginated on
either side to form paired structures, the cerebral hemispheres (Fig. 16). Ex-
cept for the corpus striatum and rhinencephalon the evaginated wall is relatively
thin, develops into the cerebral cortex, and is known as the pallium. The
lateral ventricles within the hemispheres represent portions of the original telen-
cephalic cavity and communicate with the third ventricle through the inter-
THE NEURAL TUBE AND ITS DERIVATIVES
33
ventricular foramina, which at this stage are relatively large. The lamina
terminalis, connecting the two hemispheres in front of the third ventricle, repre-
Diencephalon
Pallium
Mesencephalon
Cephalic
flexure
D
Thalamus
Pallium
\Iesencephalon
Optii
cup
Ponline flexure j
Myelencephalon ?
Meten-
cephalon
Corpus striatum.
Optic recess
Hypoihalamus
Medulla oblongata
Fig. 15. Reconstructions of the brain of a 7 mm. embryo: A, Lateral view; B, in median sagittal
section. (His, Prentiss-Arey.)
sents in a certain sense the rostral end of the brain. Immediately behind this
lamina is a portion of the telencephalic cavity which forms the anterior part of
Cerebral peduncle
Hypothalamus .^
Epithalamus \ ^aJL
Thalamnf;
Diencephalon- r
Pallium^
Telencephalon- -'
Cerebral aqueduct
[.^Mesencephalon
^^,RhombencepJialic isthmus
Cerebellum
Metencephalon
Rhomboid fossa
^Myelencephalon
1234 >
Rhinencephalon | Corpus striatum Pans
Lamina terminalis
Spinal cord
Fig. 16. A median section of the brain of a 13.6 mm. human embryo: 1, Optic recess; 2, ridge
formed by optic chiasma; 3, optic chiasma; 4, infundibular recess. (His, Sobotta.)
the third ventricle. The further development of these structures is readily
traced in Fig. 17, which represents the brain of a human fetus of the third
3
34
THE NERVOUS SYSTEM
month. The most striking feature of the brain at this stage is the great size
attained by the cerebral hemispheres.
The Diencephalon. The three principal divisions of the diencephalon
the thalamus, epithalamus, and hypothalamus faintly indicated in an embryo
Diencephalon,
Chorioid plexus
Corpus striatum
Telencephalon /
Thalamus
I Pineal body (epithalamus)
Cerebral peduncle
Cerebral aqueduct
" Mesencephalon
'-Isthmus
'*- Cerebellum
~ Metencephalon
Rhomboid fossa
Myelencephalon
: Optic Hypo-
j chiasma, physis Medulla
Lamina terminalis / "Hypothalamus blon z ata
Rhinencephalon
' Spinal cord
Central canal
Fig. 17. The brain of a fetus of the third month in median sagittal section. (His, Sobotta.)
of 13.6 mm., are well denned by the third month (Fig. 17). In transverse
sections this division of the embryonic brain is seen to be composed of a pair of
plates on either side, which with a roof and floor form the walls of the ventricle
Roof plate (with chorioid plexus)
Alar plate or Thalamus
Sulcus limitans
Basal plate or Hypothalamus
"Mammillary recess
Fig. 18. Transverse section through the diencephalon of a 13.8 mm. embryo. (His, Prentiss-
Arey.)
(Fig. 18). The dorsal lamina is known as the alar plate, the ventral as the basal
plate. On either side these meet at an angle, forming the sulcus limitans. These
laminae and the sulcus limitans between them can be traced back through the
THE NEURAL TUBE AND ITS DERIVATIVES
35
mesencephalon and rhombencephalon into the spinal cord. The thalamus is
produced by a thickening in the alar lamina and is separated from the hypo-
thalamus by the sulcus limitans, which can be traced as far as the optic recess
rostral to the ridge produced by the optic chiasma.
The hypothalamus 1 represents the basal lamina and gives rise to the tuber
cinereum, posterior lobe of the hypophysis, and the mammillary bodies. From the
dorsal edge of the alar lamina, where this is attached to the thin roof plate, there
is developed a thickened ridge, the epithalamus, which is transformed into the
habenula and the pineal body. The roof plate of the diencephalon remains
thin and forms the epithelial lining of the tela chorioidea or roof of the third
ventricle.
The Mesencephalon. The basal plate of the mesencephalon thickens to
form the cerebral peduncles (Fig. 17), the alar plate forms the lamina quad-
rigemina in which are differentiated the quadrigeminal bodies; the cavity be-
comes the cerebral aqueduct.
TABLE SHOWING SUBDIVISIONS OF THE NEURAL TUBE AND THEIR DERIVATIVES (Modified from a
Table in Keibel and Mall, Human Embryology).
Primary vehicles.
Subdivisions.
Derivatives.
Lumen.
(
Telencephalon. . . {
Cerebral cortex,
Corpora striata,
Rhinencephalon,
Lateral ventricles.
Rostral portion of
the third ventricle
(
Pars-optica hypo-
thalami.
Prosencephalon.
Diencephalon ^
Epithalamus,
Thalamus,
Hypothalamus,
Hypophysis,
The greater part of
the third ventricle.
Brain
Tuber cinereum,
Mammillary bodies,
Metathalamus.
Mesencephalon
Mesencephalon . . . . <
Corpora quadri-
gemina,
Crura cerebri.
Cerebral aqueduct.
Rhombencephalon <
Metencephalon <
Myelencephalon
Cerebellum,
Pons,
Medulla oblongata.j
Fourth ventricle.
Spinal cord
Spinal cord.
Central canal.
1 The pars optica hypothalami, including the optic chiasm, is, properly speaking, not a part
of the hypothalamus at all, but belongs to the telencephalon (Johnston, 1909, Jour. Comp. Neur ,
vol. 19, and 1912, Jour. Comp. Neur., vol. 22).
36 THE NERVOUS SYSTEM
The Rhombencephalon. The ventral part of the rhombencephalon, includ-
ing both alar and basal plates, thickens to form the pans and medulla oblongata
(Fig. 17). Most of the roof of this division remains thin and forms the epithelial
lining of the tela chorioidea of the fourth ventricle. But in the caudal portion
of the myelencephalon the lumen of the neural tube becomes completely sur-
rounded by thickened walls, forming the central canal of the closed portion of
the medulla. The posterior edge of the alar plate in the metencephalon becomes
greatly thickened and, fusing across the median line with the similar structure
of the opposite side, forms the anlage of the cerebellum (Figs. 17, 137). Later
we shall see that, in general, motor structures develop from the basal, and sen-
sory parts from the alar, plate.
The table on page 35 gives in brief the principal derivatives of the
neural tube.
CHAPTER III
HISTOGENESIS OF THE NERVOUS SYSTEM
Early Stages in the Differentiation of the Neural Tube. Hardesty (1904)
has given a good account of the early development of the spinal cord in the pig.
At first the neural plate consists of a single layer of ectodermal cells (Fig. 19, A).
These proliferate and lose their cell boundaries. When the neural tube has
closed its wall is formed of several layers of fused cells a syncytium bounded
by an external and an internal limiting membrane (Fig. 19, B, C). The syn-
cytium now becomes more open and sponge-like in structure. The nuclei are
so arranged that three layers may be differentiated: (1) an ependymal layer,
(2) a mantle layer, with many nuclei, and (3) a marginal or non-nuclear layer.
The ependymal layer is represented by a row of elongated nuclei, among which
are found the large mitotic nuclei of the germinal cells.
These germinal cells divide and give rise to ependymal cells, and to the indif-
ferent cells of the mantle layer. Through division of the latter spongioblasts
and neuroUasts are formed. From the former comes the neuroglia or supporting
tissue of the nervous system, while from the latter are derived the nerve-cells
and fibers.
The Development of the Neuron. A neuron may be defined as a nerve-
cell with all its processes; and each is derived from a single neuroblast. From
the pear-shaped neuroblast a single primary process grows out, and this be-
comes the axis-cylinder of a nerve-fiber (Fig. 20). Other processes which de-
velop later become the dendrites. The primary process, or axon, grows into the
marginal layer, within which it may turn and run parallel to the long axis of the
neural tube as an association fiber; or it may run out of the neural tube in a ven-
trolateral direction as a motor axon. In this way the motor fibers of the cere-
brospinal nerves are laid down. The axis-cylinder of each represents a process
which has grown out from a neuroblast in the basal plate of the neural tube.
Development of Afferent Neurons. The sensory or afferent fibers of the
spinal nerves take origin from neuroblasts which are from the beginning out-
side the neural tube. These neuroblasts are derived from the neural crest, a
longitudinal ridge of ectodermal cells at the margin of the neural groove, where
this becomes continuous with the superficial ectoderm. At first in contact with
37
THE NERVOUS SYSTEM
the dorsal surface of the neural tube, the neural crest soon separates from it
and comes to lie in the angle between it and the myotomes. In this position
the neural crest gives rise to a series of sensory ganglia. From neuroblasts
located in these ganglia arise the sensory fibers of the cerebrospinal nerves.
Marginal layer Mantle layer Ependymal layer
' I
.Germinal
cell
Marginal layer Ependymal layer
Mesoderm Marginal layer
\Germinal
cell
S^p
Internal limiting membrane
Ependymal layer
9
^Germinal
1 >' J cell
External limiting membrane Mantle layer Internal limiting membrane
External limiting membrane
ST 1
Germinal cell Internal limiting membrane
Mesoderm Marginal layer
Mantle layer
Ependymal layer
Fig. 19. Early stages in the differentiation of the neural tube: A, From a rabbit embryo
before closure of neural tube; B, from a 5 mm. pig embryo after closure of tube; C, from a 7 mm.
pig embryo; D, from a 10 mm. pig embryo. *, Boundary between nuclear and marginal layers.
(Hardesty, Prentiss-Arey.)
This last statement requires some qualification. The fibers of the olfactory nerve
arise from cells in the olfactory mucous membrane. The fibers of the mesencephalic root
of the trigeminal nerve, which in all probability are sensory, arise from cells located within
the mesencephalon. The optic nerve is also an exception, but this is morphologically a
fiber tract of the brain and not a true nerve. An ingenious theory, advanced by Schulte and
Tilney (1915), attempts to bring this mesencephalic root and the optic nerve into more ob-
HISTOGENESIS OF THE NERVOUS SYSTEM
39
vious relation with the other sensory nerves. They assume that the part of the neural crest,
which lies rostral to the anlage of the semilunar ganglion, fails to separate from the neural
tube. From this part of the neural crest, retained within the brain, they would derive the
mesencephalic nucleus of the trigeminal nerve and the optic vesicles.
On the other hand, there are observations which tend to show that some of the cranial
sensory ganglia are derived at least in part from other sources than the neural crest. This
is especially true of the acoustic ganglion (Streeter, 1912). According to Landacre (1910)
many of the sensory ganglion cells of the seventh, ninth, and tenth nerves are derived from
Fig. 20. A, Transverse section through the spinal cord of a chick embryo of the third day
showing neuraxons (F) developing from neuroblasts of the neural tube and from the bipolar
ganglion cells, d. B, Neuroblasts from the spinal cord of a seventy-two-hour chick. The three to
the right show neurofibrils; C, incremental cone. (Cajal, Prentiss-Arey.)
thickened patches of the superficial ectoderm, known as placodes, with which the ganglia of
these nerves come in contact at an early stage in their embryonic development. The
acoustic ganglion of the eighth nerve seems also to have a similar origin, i. e., from the cells
of the otic vesicle which is formed by a process of invagination from the superficial ectoderm.
The neuroblasts of these ganglia become bipolar through the development
of a primary process at either end (Fig. 21). Originally bipolar, a majority of
these sensory neurons in the mammal become unipolar through the fusion of
the two primary processes for some distance into a single main stem. Beyond
the point of fusion this divides like a T into two primary branches, one of which
THE NERVOUS SYSTEM
is directed centrally, the other peripherally. The centrally directed branch
grows into the neural tube as a sensory root fiber (Fig. 20, A, d) ; the other grows
peripherally as an afferent fiber of a cerebrospinal nerve. This general state-
ment requires some qualification. It may be that some bipolar neuroblasts
become unipolar by the absorption of one of the primary processes, while the
remaining one divides dichotomously into central and peripheral branches
(Streeter, 1912). It should also be noted that the cells of the sensory ganglia
of the acoustic nerve remain bipolar
throughout life.
Development of the Spinal Nerves.
We have traced the development
of the chief elements entering into
the formation of the cerebrospinal
nerves, and will now see how these
are combined in a typical spinal nerve.
The spinal ganglion, derived from
the neural crest, contains bipolar
neuroblasts, which are transformed
into unipolar neurons. The axon of
such a nerve-cell divides into a cen-
tral branch, running through the
dorsal root into the spinal cord, and
a peripheral branch, running distally
through the nerve to reach the skin
or other sensitive portion of the body.
Mingled with these afferent fibers in
the spinal nerves are efferent axons which have grown out from neuroblasts in
the basal plate of the spinal cord, througiiJJie ventral root, and are distributed
by way of the spinal nerve to muscles.
So far we have dealt only with the origin of the axis-cylinders of the nerve-
fibers. But these soon become surrounded by protective sheaths which are also
ectodermal in origin. In the path of the outgrowing axons there are seen nu-
merous spindle-shaped ectodermal cells, which have migrated from the anlage
of the spinal ganglia (Harrison, 1906), and perhaps also from the neural tube
along the ventral roots (Held, 1909). These cells form such a prominent feat-
ure in a developing nerve that some workers have thought the axons differen-
tiate in situ from them. This theory, which has been known as the cell-chain
Fig. 21. A section of a spinal ganglion from
a 44 mm. fetus, showing stages in the trans-
formation of bipolar neurons, A, into unipolar
neurons, B. Golgi method. (Cajal.)
HISTOGENESIS OF THE NERVOUS SYSTEM 41
hypothesis, and gives to each axon a multicellular origin, has been supported by
Schwann, Balfour, Dohrn, and Bethe, and in modified forms by other workers.
There are good reasons, however, for believing that each axon arises as an out-
growth from a single cell or neuroblast. This idea, which is in keeping with
what is known of the structure and function of the neuron and which forms an
integral part of the now generally accepted neuron theory, was first developed in
the embryologic publications of His. Convincing experimental evidence has
been furnished by Harrison (1906). Using amphibian larvae, this author showed
that if the neural crest and tube are removed no peripheral nerves develop.
He further showed that isolated nerve-cells cultivated in clotted lymph will
Roof plate
Dorsal column
Dorsal root
Mantle layer
Ventral column
Ependymal layer
Dorsal Juniculus
Neural cavity
Marginal layer
Floor plate Ventral median fissure
Fig. 22. Transverse section of the spinal cord of a 20 mm. human embryo. (Prentiss-Arey.)
give rise to long axons in the course of a few hours. But the ectodermal cells,
mentioned above, which migrate outward along the course of the developing
nerve, take an important part in the differentiation of the fibers. From them
is derived the nucleated sheath or neurilemma of the peripheral nerve-fiber.
The myelin sheath is composed of a fatty substance of uncertain origin. It
may be a product of the axon, of the neurilemma, or of both.
The sympathetic ganglia consist of cells derived like those of the spinal
ganglia from the neural crest, and, according to Kuntz (1910), also from the
neural tube by migration along the course of the cerebrospinal nerves. These
cells become aggregated in the ganglia of the sympathetic system and are asso-
ciated with the innervation of smooth muscle and glands.
42 THE NERVOUS SYSTEM
The spinal cord of a 20 mm. human embryo presents well-defined ependymal,
marginal, and mantle layers. Figure 22 should be compared with the appear-
ance presented by a cross-section of the spinal cord in the adult (Fig. 55). The
mantle layer with its many nuclei differentiates into the gray matter of the spinal
cord, which contains the nerve-cells and their dendritic processes. The mar-
ginal layer develops into the white substance as a result of the growth into it
of the axons from neuroblasts located within the mantle layer. These form
association fibers which ascend or descend through the marginal layer and serve
to connect one level of the neural tube with another. It is not until these
longitudinally coursing axons develop myelin sheaths that the white substance
acquires its characteristic coloration.
The cavity of the neural tube is relatively large, and at the point marked
"neural cavity" in Fig. 22 a groove is visible. This is the sulcus limitans. It
separates the dorsal or alar plate from the ventral or basal plate. The mantle
layer of the alar plate develops into the dorsal gray column which, like the other
parts developed from this plate, is afferent in function. The afferent fibers,
growing into the spinal cord from the spinal ganglia, either terminate in this
dorsal column or ascend in the posterior part of the marginal zone to nuclei
derived from the alar plate in the myelencephalon. Most of the association
fibers which run in the marginal layer have grown out from neuroblasts located
in the dorsal column. The mantle layer of the basal plate gives rise to the
ventral gray column. From the neuroblasts in this region grow out the motor
fibers of the ventral roots and spinal nerves.
From what has been said it will be clear that the entire nervous system is
ectodermal in origin. The nervous element proper or neurons are derived from
the neuroblasts; the supporting tissue of the brain and spinal cord, the neuroglia,
is derived from spongioblasts ; while the neurilemma of the peripheral nerves is
the product of sheath cells which have migrated out from the spinal ganglia
and possibly also from the neural tube.
CHAPTER IV
NEURONS AND NEURON-CHAINS
THE nervous system is composed of highly irritable cellular units, or neurons,
linked together to form conduction pathways. In the preceding chapter we
have seen that each neuron is the product of a single embryonic cell or neuro-
blast, and that, therefore, the nerve-cell with all its processes constitutes a gen-
etic unit. In the present chapter, as we examine the form and internal struc-
ture of the neurons and their relation to each other, we shall learn that they are
also the structural and functional units of the nervous system.
Form. There is the widest possible variation in the shape of nerve-cells,
but all present some features in common. About the nucleus there is an accumu-
lation of cytoplasm which together with the nucleus forms what is often called
the cell body. A convenient term by which to designate the circumnuclear
cytoplasmic mass is perikaryon. From the perikaryon cytoplasmic processes
are given off, some of which may be of great length. The external form of
the neuron depends on the shape of the perikaryon and on the number, shape,
and ramification of these processes. Since the variety of forms is almost with-
out limit, we will content ourselves with studying a few typical examples.
The pyramidal cells of the cerebral cortex are good examples (Fig. 23). The
perikaryon is triangular in form. One angle, that directed toward the surface
of the cortex, is prolonged in the form of a long thick branching process, the
apical dendrite. From the sides and other angles of the perikaryon arise shorter
branching dendrites, while from the base or from one of the basal dendrites
arises a long slender process, the axon. The characteristic features of the den-
drites are as follows: they branch repeatedly, rapidly decrease in size, and
terminate not far from the cell body. Their contour is irregular and they are
studded with short side branches, or gemmules, which give them a spiny appear-
ance. Each neuron usually possesses several dendrites, but in some types of
nerve-cells they are absent altogether. The axon, on the other hand, is char-
acterized by its uniform smooth contour, relatively small diameter, and in most
instances by its great length and relative freedom from side branches. It may
give off fine side branches, or collaterals, near its origin; and these arise at right
43
44
THE NERVOUS SYSTEM
angles to the parent stem. The axon
terminates in a multitude of fine branches
usually at a considerable distance and
sometimes as much as a meter from its
origin. The origin of the axon from the
perikaryon is marked by an expansion
known as the cone of origin or im-
plantation cone. This cone, like the
axon, differs somewhat in structure from
the perikaryon. Such long axons as
have just been described are character-
istic of the cells of Golgi's Type I.
That not all axons are long and
relatively unbranched is seen from Fig.
24, which illustrates a cell of Golgi's
Type II. The axons of these cells are
short, branch repeatedly, and end in the
neighborhood of the cell body.
Another good example is furnished
by the primary motor neurons. Figure
25 illustrates such a cell from the anterior
gray column of the spinal cord. This
is a large nerve-cell with many rather
long branching dendrites and an axon,
which forms the axis-cylinder of a motor
nerve-fiber and terminates by forming
a motor ending in a muscle. As illus-
trated in this figure, long axons tend to
acquire myelin sheaths, and those
which run in the cerebrospinal nerves
are also covered by a nucleated mem-
branous sheath the neurilemma.
Nerve-cells with many processes,
such as have just been described, are
called multipolar. Examples of unipo-
lar and bipolar cells are furnished by the
cerebrospinal ganglia (Fig. 40). These cells, which will be described in more
Fig. 23. A pyramidal cell from the cere-
bral cortex of a mouse : a, Dendrites from the
base of the cell; b, white substance of the
hemisphere into which the axon, e, can be
traced; c, collaterals from the first part of the
axon; /, apical dendrite; p, its terminal
branches near the surface of the cortex.
Golgi method. (Cajal.)
NEURONS AND NEURON-CHAINS
45
detail in another chapter, are devoid of dendrites. The axon of such a unipolar
cell divides dichotomously into a central and a peripheral branch, each possess-
ing the characteristics of an axon.
It is not uncommon to regard the peripheral branch of a sensory neuron as a dendrite,
because like the dendrites it conducts nerve impulses toward the cell body. But, since it
possesses all the morphologic characteristics of an axon, and since any axon is able to con-
duct nerve impulses throughout its length in either direction, and since these peripheral
branches of the sensory neurons actually convey impulses distally in the phenomenon of
Fig. 24. Neurons with short axons (Type II of Golgi) from the cerebral cortex of a child: a,
Axon. Golgi method. (Cajal.)
antidromic conduction (Bayliss, General Physiology, p. 474), it seems best to consider both
central and peripheral branches as divisions of a common axonic stem. (See Barker, The
Nervous System, p. 361.)
From what has been said it will be apparent that a neuron usually possesses
several dendrites and a single axon, but some have only one process, which is
then an axon. It may be added that some neurons have more than one axon.
Nerve-fibers are axons naked or insheathed. Two myelinated peripheral
nerve-fibers are shown in Fig. 26. The axon or axis-cylinder is composed of
THE NERVOUS SYSTEM
delicate neurofibrils embedded in a semifluid neuroplasm. It is surrounded by
a relatively thick myelin sheath and a nucleated membranous neurilemma sheath.
Fig. 25. Primary motor neuron (diagram-
matic): ah, Implantation cone of axon; ax,
axon; c, cytoplasm; d, dendrites; m, myelin
sheath; m', striated muscle; n, nucleus; ri,
nucleolus; nR, node of Ranvier; sf, collateral;
si, neurilemma ; tel, motor end-plate. (Barker,
Bailey.)
Fig. 26. Portions of two nerve-fibers
stained with osmic acid (from a young rabbit).
Diagrammatic. 425 diameters: RR, Nodes of
Ranvier, with axis-cylinder passing through; a,
neurilemma; c, opposite the middle of the seg-
ment, indicates the nucleus and protoplasm ly-
ing between the neurilemma and the medullary
sheath. In A the nodes are wider, and the in-
tersegmental substance more apparent than in
B. (Schafer, in Quain's Anatomy.)
The myelin sheath consists of a fatty substance, myelin, supported by a retic-
ulum of neurokeratin. The latter, not seen in the living fiber, may be a coag-
ulation product produced during fixation. The highly refractive myelin gives
NEURONS AND NEURON-CHAINS
47
to the myelinated fibers a whitish color. This sheath is interrupted at regular
intervals by constrictions in the nerve-fiber known as the nodes of Ranvier.
The constrictions are produced by a dipping in of the neurilemma sheath toward
the axon, which runs without interruption through the node. The part of a
fiber between each node is an internodal segment, and each such segment pos-
sesses a nucleus which is surrounded by a small amount of cytoplasm and lies
just beneath the neurilemma. The latter is a thin membranous outer covering
for the fiber. Each segment of the neurilemma sheath, together with the cell
which lies beneath, is the product of a single sheath cell of ectodermal origin.
Fibers such as have just been described are found in the cerebrospinal nerves,
and give these their white glistening appearance.
The myelinated filers of the brain and spinal cord are of somewhat different
structure. There is no evidence of segmentation in the myelin sheath and
neither the neurilemma nor its cells are present. This fact is of much im-
portance in the phenomena of regeneration, as will be explained later. These
are the fibers which give the characteristic color to the white matter of the
brain and spinal cord.
Unmyelinated fibers are of two kinds, namely, Remak's fibers and naked ,
axons. The former possess nuclei which may be regarded as belonging to a
thin neurilemma. They are found in great numbers in the sympathetic nervous
system, and many of the fine afferent fibers of the cerebrospinal nerves also
belong to this class (Ranson. 1911). Naked axons are especially numerous in
the gray matter of the brain and spinal cord, and it may be added that every
axon at its beginning from the nerve-cell, as well as at its terminal arborization,
is devoid of covering.
By way of summary we may enumerate four kinds of nerve-fibers: (1) myelin-
ated fibers with a neurilemma, found in the peripheral nervous system, especially
in the cerebrospinal nerves; (2) myelinated fibers without a neurilemma, found
in the central nervous system; (3) unmyelinated fibers with nuclei (Remak's
fibers), especially numerous in the sympathetic system, and (4) naked axons,
abundant in the gray matter of the brain and spinal cord.
Neuroglia cells and fibers will be considered in connection with the structure
of the spinal cord.
Structure of Neurons. Like other cells, a neuron consists of a nucleus sur-
rounded by cytoplasm, and these possess the fundamental characteristics which
belong to nucleus and cytoplasm everywhere, but each presents certain features
more or less characteristic of the nerve-cell. The nucleus is large and spheric;
48 THE NERVOUS SYSTEM
and, because it contains little chromatin, it stains lightly with the basic dyes
(Fig. 27, A). It contains a large spheric nucleolus. The cytoplasm, enclosed
in a cell membrane, is characterized by the presence of basophil granules and
a fibrillar reticulum. The granules, which apparently are a product of the
nucleus, are composed of nucleoprotein. They are grouped in dense clumps,
known as Nissl bodies or tigroid masses, and stain deeply with methylene-blue.
The size, shape, and arrangement of the Nissl bodies differ with the type of
nerve-cell studied. They are much larger in motor than in sensory neurons
(Malone, 1913). While they are found in the larger dendrites, the axon and
its cone of origin are free from them. They are intimately concerned in the
Axon
Fig. 27. Nerve-cells stained with toluidin blue: A, From anterior horn of spinal cord of the
monkey, shows Nissl bodies in cytoplasm; B, from the facial nucleus of a dog, shows a partial
disappearance of the Nissl bodies (chromatolysis) resulting from section of the facial nerve.
(Schafer.)
metabolic activity of the cell, increasing during rest and decreasing as a result
of fatigue. They also undergo solution as a result of injury to the axon even
at a great distance from the cell, the so-called axon-reaction or chromatolysis
(Fig. 27, B).
The neurofibrils were first brought forcefully to the attention of neurologists
by Bethe (1903). These are delicate threads which run through the cytoplasm
in every direction and extend into the axon and dendrites (Fig. 28). The
appearance of the fibrillae differs according to the technic employed in preparing
the tissue for microscopic examination. While in the preparations by Bethe's
method the fibrils do not appear to branch or anastomose with each other, those
seen in Caial preparations divide, and by anastomosing with each other form
NEURONS AND NEURON-CHAINS
49
a true network. The fibrillge can be traced to the terminations of the dendrites
and axons. They have been looked upon by many as the chief elements in-
volved in the conduction of the nerve impulse.
Other elements such as pigment granules may be present. Mitochondria
have been described in nerve-cells by Cowdry (1914) and Rasmussen (1919).
Interrelation of Neurons. In the
ccelenterates, as we have learned, a single
nerve-cell may receive the stimulus and
transmit it to the underlying muscle.
But in vertebrates the transmission of a
nerve impulse to an effector requires
a chain of at least two neurons, the im-
pulse passing from one neuron to the next
along the chain. One of the most im-
portant problems in neurology, there-
fore, is this: How are the neurons re-
lated to each other so that the impulse
may be propagated from one to the
other? The place where two such units
come into such functional relation is
known as a synapse. In a synapse the
axon of one neuron terminates on the
cell body or dendrites of another. Func-
tional connections are never established
between the dendrites of one neuron
and the cell body or dendrites of an-
other. In Fig. 29 the axon of a basket
cell of the cerebellum is seen giving off
collaterals which terminate about and
form synapses with the Purkinje cells.
Another type of synapse is illustrated in
Fig. 70.
The processes of one nerve-cell are not directly fused with those of others,
but, on the contrary, each neuron appears to be a distinct anatomic unit. At
least the most detailed study of Golgi and Cajal preparations, in which the
finest ramifications of dendrites and axons are stained, has failed to demon-
strate a structural continuity between neurons. In especially favorable material
Fig. 28. Neurofibrils in a cell from the
anterior gray column of the human spinal
cord: ax, Axon; lii, interfibrillar spaces; n,
nucleus; x, neurofibrils passing from one
dendrite to another; y, neurofibrils passing
through the body of the cell. (Bethe, Hei-
denhain.)
50 THE NERVOUS SYSTEM
Bartelmez (1915) has shown that an axon and dendrite, entering into the forma-
tion of a synapse, are each surrounded by a distinct plasma membrane and
that there is no direct protoplasmic continuity. It has been maintained by
Bethe and others that at such points of contact the neurofibrils pass without
interruption from one neuron to another, but this has been denied by Cajal.
The relation between two neurons at a synapse appears to be one of contact,
but not of continuity of substance.
Nerve impulses pass across the synapse in one direction only, i. e., from the
axon to the adjacent cell body or dendrite. As a corollary of this it is obvious
that impulses must travel within the neuron from dendrites to perikaryon and
then out along the axon, as indicated by the arrow in Fig. 30. This is known
Fig. 29. Basket cell from the cerebellar cortex of the white rat. The Purkinje cells are indicated
in stipple. Golgi method. (Cajal.)
as the law of dynamic polarity. The polarity is, however, not dependent upon
anything within the neuron itself, but upon something in the nature of the
synaptic interval which permits the impulses to travel across it in one direc-
tion only. There are many lines of evidence which indicate that when once
activated a nerve-fiber conducts equally well in either direction. When a motor
fiber bifurcates, sending a branch to each of two separate muscles, stimulation
of one branch will cause an impulse to ascend to the point of bifurcation, and
then descend along the other branch to its motor ending (Fig. 30). This can
often be demonstrated in regenerated nerves (Feiss, 1912). The phenomena
of antidromic conduction and the axon reflex (Bayliss, 1915) are also explained
by the assumption that impulses are able to travel along a nerve-fiber in either
direction.
NEURONS AND NEURON-CHAINS
The Neuron as a Trophic Unit. All parts of a cell are interdependent, and a
continuous interaction between the nucleus and cytoplasm is a necessary con-
dition for life. Any part which is detached from the portion containing the
nucleus will disintegrate. In this respect the nerve-cell is no exception. When
an axon is divided, that part which is separated from its cell of origin and
therefore from its nucleus dies, while the part still connected with the cell
usually survives. The degeneration of the distal fragment of the axon extends
to its finest ramifications, but does not pass the synapse nor involve the next
neuron.
It must not be supposed, however, that the part of the neuron containing the
nucleus remains intact, for as a result of the division of an axon important
Motor neuron
Synapse
Sensory neuron
Fig. 30. Diagram of a reflex arc to illustrate the law of dynamic polarity. The arrows indicate
the direction of conduction.
changes occur in the cell body. The Nissl bodies undergo solution, the cell
becomes swollen, and the nucleus eccentric. This phenomenon is known as
chromatolysis, or the axon reaction, and is illustrated in Fig. 27, B. If the
changes have been very profound the entire neuron may completely disin-
tegrate; but, as a rule, it is restored to normal again by reparative processes.
The nucleus becomes more central, the Nissl bodies reform and usually become
more abundant than before, while from the cut end of the axon new sprouts
grow out to replace the part of the axon which has degenerated. From what
has been said it will be apparent that the nucleus presides over the nutrition of
the entire neuron, that the latter responds as a whole to an injury of even a
distant part of its axon, that the changes produced by such a lesion are limited
to the neuron directly involved, and that nerve-fibers are unable to maintain
52 THE NERVOUS SYSTEM
a separate existence or to regenerate when their continuity with the cell body
has been lost. This is what is meant by the statement that the neuron is the
trophic unit of the nervous system.
Degeneration and Regeneration of Nerve-fibers. As has already been stated,
that portion of a divided fiber which has been separated from its cell of origin
degenerates. The axon breaks up into granular fragments, the myelin under-
goes chemical change and forms irregular fatty globules. Later the degenerated
axon and myelin are entirely absorbed. The neurilemma cells of a degener-
ated peripheral nerve-fiber increase in number, their cytoplasm increases in
quantity, and they become united end to end to form nucleated protoplasmic
bands or band-fibers. These changes in the nerve-fiber are known as Wallerian
degeneration.
In regeneration new axons grow out from the old ones in the central unde-
generated portion of the nerve. These grow into the distal degenerated stump
and find their way along the nucleated protoplasmic bands, mentioned above,
to the terminals of the degenerated nerve. These band-fibers serve as conduits
for the growing axons and from them the new neurilemma sheaths are differ-
entiated. Thus, while the neurilemma cells and the band-fibers derived from
them appear to be incapable of developing new nerve-fibers by themselves in
the peripheral stump, they play an important part in nerve regeneration in
co-operation with the new axons from the central stump (Cajal, 1908; Ranson,
1912). It is important to note that the nerve-fibers of the brain and spinal
cord, which, as has been stated before, are devoid of neurilemma sheaths, are
incapable of regeneration.
The neuron concept, which is based on such facts as have been presented
in the preceding paragraphs, was first clearly formulated by Waldeyer in 1891,
who was also the first to use the name neuron for the elements under considera-
tion. The neuron doctrine may be summarized as follows:
1. The neuron is the genetic unit of the nervous system each being derived
from a single embryonic cell, the neuroblast.
2. The neuron is the structural unit of the nervous system, a nerve-cell with
all its processes. These cellular units remain anatomically separate, i. e., while
they come into contact with each other at the synapses there is no continuity
of their substance.
3. The neurons are the functional units of the nervous system. They are
conduction units and the conduction pathways are formed of chains of such
units.
NEURONS AND NEURON-CHAINS
53
4. The neuron is also a trophic unit, as is seen (a) in the degeneration of a
portion of an axon severed from its cell of origin, (6) in the phenomenon of
chromatolysis or axon reaction, and (c) in the regeneration of the degenerated
portion of the axon by an outgrowth from that part of the axon still in con-
tact with its cell of origin.
5. Neurons are the only elements concerned in the conduction of nerve
impulses. The nervous system is composed of untold numbers of such units
linked together in conduction systems.
While a majority of neurologists now accept the neuron doctrine as pre-
sented here, there are dissenters (Marui, 1918). In his very interesting book,
Fig. 31. Diagrammatic section through the spinal cord and a spinal nerve to illustrate a
simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron;
/, commissural neuron.
"Allgemeine Anatomic und Physiologic des Nervensystems," Bethe has vigor-
ously controverted every one of the five cardinal points just presented.
We will next examine some of the simpler chains of neurons to see how they
enter into the formation of the conduction pathways.
Neuron-chains. The simplest functional combination of neurons is seen in
the reflex arc, and this again in its simplest form is illustrated in Fig. 31. Such
an arc may consist of but two neurons, one of which is afferent and conducts
toward the spinal cord; the other is efferent and conducts the impulses to the
organ of response. The arc consists of the following parts: (1) the receptor,
the ramification of the sensory fiber in the skin or other sensory end organ;
(2) the first conductor, which includes both branches of the axon of the spinal
ganglion cell; (3) a center including the synapse; (4) the second conductor, which
54
THE NERVOUS SYSTEM
includes the entire motor neuron, with its cell body in the anterior gray column
and its motor ending on the muscle, and (5) the effector or organ of response,
which in this case is a muscle-fiber. A wave of activation, known as the nerve
impulse, is developed in the sensitive receptor, travels over this arc, and on
reaching the muscle causes it to contract. So simple a reflex is rare, but prob-
ably the knee-jerk is an example (Jolly, 1911). A more common form of reflex
arc involves a third, and purely central neuron, as illustrated on the right side
of Fig. 31. Such central elements may be spoken of as association and com-
missural neurons. Many of them serve to connect distant parts of the central
Fig. 32. Diagram representing some of the conduction paths through the mammalian central
nervous system. An elaborate system of central or association neurons furnishes a number of
alternative paths between the primary sensory and motor neurons. (Redrawn from Bayliss.)
nervous system with each other (Fig. 68). It is to the multiplication of these
central neurons that we owe the complicated pathways within the mammalian
brain and spinal cord.
Pathways Through Higher Centers. A good idea of how the neurons of some
of the centers in the brain are related to the primary motor and sensory spinal
neurons is given by Fig. 32. It will be seen that many paths are open to an
impulse entering the spinal cord by way of a dorsal root fiber: (1) It may pass
by way of a collateral to a primary motor neuron in a two-neuron reflex arc.
It may travel over an association neuron, belonging (2) to the same level of the
NEURONS AND NEURON-CHAINS 55
spinal cord, or (3) to other levels, in reflex arcs of three or more neurons each;
or (4) it may ascend to the brain along an ascending branch of a dorsal root
fiber. Here it may travel over one or more of a number of paths, each con-
sisting of several neurons, and be finally returned to the spinal cord and make
its exit by way of a primary motor neuron. The figure illustrates but a few of
the possible paths, many of which we shall have occasion to consider in the
subsequent chapters.
For an incoming impulse a variety of paths are open, one or more of which
may be taken according to the momentary resistance of each. There is reason
to believe that the resistance to conduction across a synapse may vary from
moment to moment, according to the physiologic state of the neurons involved.
It is therefore not necessary that every impulse entering by a given fiber shall
travel the same path within the central nervous system nor produce the same
result. The pathways themselves are, however, more or less fixed, and depend
upon the structural relations established among the neurons. Many of these
synaptic connections are formed before birth, follow an hereditary pattern,
and are approximately the same for each individual of the species. In the child
these are illustrated by the nervous mechanisms involved in breathing and
swallowing, which are perfect at birth. The newly hatched chick is able to run
about and pick up food, acts which are dependent on nervous connections al-
ready established according to hereditary pattern. In man and to a less extent
in other mammals the nervous system continues to develop long after birth.
This postnatal development is influenced by the experience of the individual
and is more or less individual in pattern. It is probable "that in certain parts
of the nervous mechanism new connections can always be established through
education" (Edinger, 1911).
The neurons which make up the nervous system of an adult man are there-
fore arranged in a system the larger outlines of which follow an hereditary pat-
tern, but many of the details of which have been shaped by the experiences of
the individual.
CHAPTER V
THE SPINAL NERVES
WE have had a glance at the earliest beginnings of a nervous system in the
animal series and learned something of its biologic significance. We have
traced briefly its development in the mammalian embryo, and become familiar
with its chief subdivisions. We have studied the microscopic units of which it
is composed, learning something of their development, structure, and function.
With this information we are prepared to take up a more detailed study of the
various subdivisions of the system.
Subdivisions of the Nervous System. The most convenient and logical
classification of the parts of the nervous system is that which emphasizes the
distinction between the central organs and those peripheral portions which are
concerned chiefly in conducting impulses to and from the central organs, as
follows :
The central nervous system:
Brain,
Spinal cord.
The peripheral nervous system :
Cerebrospinal nerves:
Cranial nerves,
Spinal nerves.
The sympathetic nervous system.
The anatomic relationships of these subdivisions in man are illustrated in
Figs. 33 and 34. The brain lies within and nearly fills the cranial cavity. It is
continuous through the foramen magnum with the spinal cord, which occupies
but does not fill the vertebral canal. From the brain arises a series of nerves
usually enumerated as twelve pairs and known as cranial or cerebral nerves;
while thirty-one pairs of segmentally arranged spinal nerves take origin from the
spinal cord.
Branches of the cerebrospinal nerves reach most parts of the body. They
are composed of afferent fibers, which receive and carry to the central nervous
system sensory impulses produced by external or internal stimuli, and of efferent
fibers, which convey outgoing impulses to the organs of response. It is through
56
THE SPINAL NERVES
57
the central nervous system that the incoming impulses find their way into the
proper outgoing paths. To bring about this shunting of incoming impulses
into the appropriate efferent paths requires the presence of untold numbers
Ciliary ganglion Maxillary nerve
Sphenopalaline ganglion ,
Superior cervical ganglion of sympathetic \ \
Cervical
plexus
Brachial f
plexus |
Greater
splanchnic nerve
Lesser
splanchnic nerve
Sacral
plexus
Pharyngeal plexus
Middle cervical ganglion of sympathetic
Inferior cervical gang, of sympathetic
Recurrent nerve
Bronchial plexus
Cardiac plexus
Esophageal plexus
Coronary plexus
Left vagus nerve
Gastric plexus
Celiac plexus
Superior mesenteric plexus
Aortic plexus
Inferior mesenteric plexus
Hypogastric plexus
Pelvic plexus
Bladder
Vesical plexus
Fig. 33. Fig- 34-
Fig. 33. General view of the central nervous system, showing the brain and spinal cord in situ.
(Bourgery, Schwalbe, van Gehuchten.)
Fig. 34. Diagram of the sympathetic nervous system and its connections with the cerebrospmal
nerves. (Schwalbe, Herrick.)
of central or association neurons, and it is of these that the central organs-
brain and spinal cord are chiefly composed.
Many authors employ a classification which emphasizes the distinction be-
5 THE NERVOUS SYSTEM
tween the cerebrospinal nervous system, composed of the brain and spinal cord
with their associated nerves, and the sympathetic nervous system. But this usage
has the disadvantage that it is likely to engender an entirely false notion of the
independence of the sympathetic system.
The spinal nerves take origin from the spinal cord within the vertebral canal
and make their exit from this canal through the corresponding intervertebral
foramina. As component parts of such a nerve there may be recognized a
ventral and a dorsal ramus, a ventral and a dorsal root, and associated with
the latter a spinal ganglion. The fibers of the ventral root have their cells of
origin within the spinal cord and are distributed through both ventral and
dorsal rami. Since they conduct impulses from the spinal cord they are known
as efferent or motor fibers. The sensory or afferent fibers of the dorsal roots
and spinal nerves arise from cells located in the spinal ganglia. These fibers
are also distributed through both ventral and dorsal rami (Fig. 37).
Metamerism. That the spinal nerves are segmentally arranged, a pair for
each metamere, is readily appreciated in the case of the typical body segments
of the thoracic region. Here it is obvious that a nerve supplies the correspond-
ing dermatome and myotome, or in the adult the skin and musculature of its
own segment. While the thoracic nerves retain this primitive arrangement in
the adult, the distribution of fibers from the other spinal nerves is complicated
by the development of the limb buds and by the shifting of myotomes and
dermatomes during the development of the embryo.
Opposite the attachment of the limb buds the ventral rami of the correspond-
ing nerves unite to form flattened plates, and from these plates the brachial and
lumbosacral plexuses are developed. Within these plexuses the fibers derived
from a number of ventral rami are intermingled in what appears at first to be
hopeless confusion. Each nerve which extends from these plexuses into the
limbs carries with it fibers from more than one spinal nerve. To determine
the exact distribution of the fibers from each segmental nerve has been a very
difficult problem, in the elucidation of which the work of clinical neurologists
has been of the first importance. A study of the paralyses and areas of anes-
thesia, resulting from lesions involving one or more nerve roots within the ver-
tebral canal, has contributed much toward its solution.
Sherrington (1894) attacked the problem of the distribution of the sensory
fibers by experimental methods on cats and monkeys. He found that section of
a single dorsal root did not cause complete anesthesia anywhere, and attributed
this result to an overlapping of the areas of distribution of adjacent spinal nerves.
THE SPINAL NERVES
59
Next, selecting a particular dorsal root for study, he cut two or three roots
both above and below it. The zone in which sensation still existed and which
was surrounded by an area of anesthesia represented the cutaneous field of that
particular root. He found that each "sensory root field" overlapped those of
adjacent roots (Fig. 35). In the thoracic region each such field has the shape of
a horizontal band wrapping half-way around the body from the middorsal to
the midventral lines (Fig. 36).
Sherrington also found that, although in the plexuses associated with the
innervation of the extremities each segmental nerve contributes sensory fibers
to two or more peripheral nerves, the cutaneous distribution of these fibers is
not composed of disjointed patches, but forms a continuous field running approxi-
mately parallel to the long axis of the limb. The general arrangement of these
sensory root fields in man is indicated on the right side of Fig. 36. On the
Uth
thoracic
sensory <
spinal
^ kin field.
///////////////////////////////I////
//////////// luiin iniini a 1 1 n
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
J\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
I
\
3d
thoracic.
5th
thoracic.
Fig. 35. Diagram of the position of the nipple in the sensory skin fields of the fourth, third,
and fifth thoracic spinal roots. The overlapping of the cutaneous areas is represented. (Sher-
rington.)
opposite side is indicated the distribution of the cutaneous nerves. It will be
seen that in the extremities there is no correspondence between the areas sup-
plied by these peripheral nerves and those supplied by the individual dorsal
roots. It will also be evident that the fibers of a given dorsal root reach the
corresponding sensory root field by way of more than one cutaneous nerve.
A knowledge of the cutaneous distribution of the various nerve roots is of great
importance in enabling the clinician' to determine the level of a lesion of the
spinal cord or nerve roots within the vertebral canal.
In the same way the shifting of muscles during embryonic development has
been accompanied by corresponding changes in the spacial distribution of the
motor fibers. A familiar example is furnished by the diaphragm, the musculature
of which is derived from the cervical myotomes and which in its descent carries
with it the phrenic nerve. This explains the origin of the phrenic from the
third, fourth, and fifth cervical nerves.
6o
THE NERVOUS SYSTEM
If, as seems probable, the musculature of the extremities has not developed
along metameric lines, there can be no true metamerism of the motor nerves to
the limbs (Streeter, 1912). Yet the fibers from each ventral root are distributed
in a very orderly manner. As is indicated in the table on page 77, almost every
long muscle receives fibers from two or more ventral roots. It will be apparent
that the muscles of the trunk are innervated from the roots belonging to the
Great auricular
Cutaneous nerve of the neck
Supraclavicular nerves
Axillary
Intercostobrachial
Medial cutaneous of arm
Posterior cutaneous of arm
Medial cutaneous of forearm
Musculocutaneous
Radial
Median
Ulnar
Genitofemoral '
Lateral cutaneous of the thigh'
Intermediate cutaneous rami'
Medial cutaneous rami
Infrapatellar ramus
Lateral sural
Saphenous
Superficial peroneal
Sural
Deep peroneal
Fig. 36. Sensory root fields on the right, contrasted with the areas of distribution of cutaneous
nerves on the left.
several metameres from the myotomes of which these muscles * developed. The
table shows in a general way the distribution of the fibers of the several ventral
roots.
Functional Classification of Nerve-fibers. Many years ago Sir Charles
Bell (1811, 1844) showed that the dorsal roots are sensory in function and the
ventral roots motor; and this has been known since then as Bell's law. He
recognized that sensory and motor fibers are distributed to the viscera as well as
THE SPINAL NERVES
6l
to the rest of the body. But GaskeU (1886) was the first to make a detailed
study of the nerve-fibers supplying the visceral and vascular systems. We
now recognize in the spinal nerves elements belonging to four functionally
distinct varieties, namely, visceral afferent, visceral efferent, somatic afferent, and
somatic efferent fibers (Fig. 37).
Visceral Components. The fibers which innervate the visceral and vascular
systems, including all involuntary muscle and glandular tissue, possess, as
Gaskell (1886) pointed out many years ago, certain distinguishing character-
istics. They are all fine myelinated fibers and end in sympathetic ganglia
Somatic afferent fiber \ Dorsal root
Visceral afferent fiber)
Spinal ganglion
Dorsal ramus
f Ventral ramus
|
" Ramus communicans
Sympathetic ganglion
/*. Visceral efferent fiber], , . ,
S Somatic efferent fiber Central root
Postganglionk fiber
,Viscus
Fig. 37. Diagrammatic section through a spinal nerve and the spinal cord in the thoracic region
to illustrate the chief functional types of peripheral nerve-fibers.
from which the impulses are relayed to involuntary muscles and glands by a
second set of neurons (Fig. 37). They are usually designated as visceral efferent
fibers, and they run by way of the white rami to the sympathetic ganglia. It
is usually stated that they are found only hi the second thoracic to the second
lumbar nerves inclusive, but Langley (1892) has shown that in the cat, dog,
and rabbit they are present in all the thoracic and the first four lumbar nerves,
and Miiller (1909) found white rami associated with the third and fourth lumbar
nerves in man.
There are also visceral afferent fibers distributed to the thoracic and ab-
dominal viscera by way of the white rami from the thoracic and upper lumbar
62
THE NERVOUS SYSTEM
nerves. These have their cells of origin in the spinal ganglia and are continued
through the dorsal roots into the spinal cord (Fig. 37). We shall have much
more to say about the visceral components of the spinal nerves in the chapter
on the Sympathetic Nervous System. In the remaining pages of this chapter
we will confine our attention to the somatic components, i. e., to those fibers which
innervate the various parts of the body exclusive of the visceral and vascular
systems.
Somatic Efferent Components. The skeletal muscles are innervated by
myelinated fibers, which are, for the most part, of large caliber. The axis-
cylinders of these fibers are the axons of cells located in the ventral part of the
gray matter of the spinal cord, and they end on the muscle-fibers in special
Fig. 38. Nerve-ending in muscular fiber of a lizard (Lacerta viridis). Highly magnified: a,
End-organ seen in profile; b, from the surface; s, s, sarcolemma; p, p, expansion of axis-cylinder.
Beneath this is granular protoplasm containing a number of large clear nuclei and constituting
the "bed" or "sole" of the end-organ. In b the expansion of the axis-cylinder appears as a clear
network, branching from the divisions of the medullated fiber. (Kiihne in Quain's Anatomy.)
motor end-plates. Such a primary motor neuron is illustrated in Fig. 25. A
motor fiber undergoes repeated division as it approaches its termination, but
each branch retains its myelin sheath until in contact with the muscle-fiber.
At this point this sheath terminates abruptly, and the neurilemma becomes
continuous with the sarcolemma (Fig. 38). The terminal branches of the
axon are short, thick, and irregular. They lie immediately under the sarcolemma
in a bed of specialized sarcoplasm containing a number of large clear nuclei.
The wave of activation, which travels down an axon as a nerve impulse, is
transmitted through these motor nerve endings to the muscle and initiates a
contraction.
The Spinal Ganglia. Since the afferent fibers in the spinal nerves take their
THE SPINAL NERVES 63
origin from the ganglia on the dorsal roots we will do well to interrupt for a
moment our functional analyses of the spinal nerves and consider the struc-
ture of these ganglia.
The spinal ganglia are rather simple structures so far as their fundamental
plan is concerned, but in recent years, chiefly through the studies of Cajal
(1906) and Dogiel (1908), we have learned to recognize in them many complex
histologic details, the significance of which is not yet understood. It has long
been known that the typical cells of the mammalian spinal ganglion are uni-
polar. The cell body is irregularly spheric. The axon, 1 which is attached to
the perikaryon by an implantation cone, is coiled on itself in the neighborhood
of the cell, forming what is known as a glomerulus (Fig. 39, /). It then runs
into one of the central fiber bundles of the ganglion and divides in the form
of a T or Y into two branches, of which one is directed toward the spinal cord
in the dorsal root. The other and somewhat larger branch is directed distally
in the spinal nerve. The cells vary greatly in size and the diameter of the axon
varies with that of the cell from which it springs. An axon arising from a
large cell usually forms a very pronounced glomerulus and soon becomes en-
sheathed with myelin, and this myelin sheath is continued along both branches
into which it divides. The branching occurs at a node of Ranvier.
As was originally pointed out by Cajal (1906) and Dogiel (1908) and
recently emphasized by Ranson (1911) the small cells of these ganglia give rise
to fine unmyelinated fibers. These coil but little near the cell, or the glomerulus
may be entirely lacking (Fig. 39, a). They divide dichotomously, just as do
the myelinated fibers, into finer central and coarser peripheral branches. At
the point of bifurcation there is a triangular expansion in place of the constric-
tion so characteristic of a dividing myelinated fiber. It has been shown by
Hatai (1902) and Warrington and Griffith (1904) that the small cells are con-
siderably more numerous than the large cells, though because of their small
size they constitute a less conspicuous element.
A few cells retain the bipolar form characteristic of all the spinal ganglion
cells at an early stage of development (Figs. 21, 40, d).
The spinal ganglion cells are each surrounded by a capsule or membranous
sheath with nuclei on its inner surface (Fig. 39, d, /) which is continuous with
the neurilemma sheath of the associated nerve-fiber. The cells forming the
capsule are of ectodermal origin, being derived like the spinal ganglion cells
themselves from the neural crest.
1 See fine print, page 45.
04 THE NERVOUS SYSTEM
In good methylene-blue preparations and in sections stained by the newer
silver methods it is possible to make out many additional details of structure.
The axon may split into many branches, which subdivide and anastomose,
forming a true network in the neighborhood of the cell (Fig. 39, b). From this
network the axon is again assembled and passes on to a typical bifurcation.
Or the axon may be assembled out of a similar plexus which, however, is con-
&.
a
Fig. 39. Neurons from the spinal ganglion of a dog: a, Small cells with unmyelinated axons;
6, c, d, e, and /, large cells with myelinated axons; /, typical large spinal ganglion cell showing
glomerulus and capsule. The arrow points toward the spinal cord. Pyridin-silver method.
nected with the cell by several roots (Fig. 39, c}. Some of the fibers give off
collaterals terminating in spheric or pear-shaped end-bulbs. Such an end bulb
may rest upon the surface of its own perikaryon (Fig. 39, d] or elsewhere in the
ganglion. From the body of some cells short club-shaped dendrites arise, which,
however, terminate beneath the capsules which surround the cells.
Based on such details as these Dogiel (1908) has arranged the spinal ganglion cells in
groups and recognizes eleven different types. Two of his eleven types are of special interest.
The cells of Type VIII resemble the typical spinal ganglion cell in all respects except that
THE SPINAL NERVES 65
the peripheral branch of the axon breaks up within the ganglion into numerous myelinated
fibers, which after losing their sheaths terminate in what are apparently sensory endings.
The central branch runs apparently without division to the spinal cord. The cells of Type
XI possess, in addition to an axon, that apparently runs without division through the dorsal
root to the spinal cord, several processes that resemble dendrites, in that they divide re-
peatedly within the ganglion, but resemble axons in their appearance and in possessing
myelin sheaths (Fig. 40, b). These processes after repeated divisions become unmyelinated
and end within the ganglion and dorsal root in what appear to be sensory endings. It would
lead us too far afield if we should attempt to summarise Dogiel's work. It should be pointed
out, however, that he no longer believes in the existence of the cells which he formerly de-
scribed under the head of spinal ganglion cells of Type II and which find a conspicuous place
in most text-books. He believes that what he formerly described as the branching fibers
of these cells are, in reality, the dendrite-like branches of the cells of Type XI.
Dorsal root /,
Dorsal ramus
Ventral root
Ramus communicans
Ventral ramus
Fig. 40. Diagrammatic longitudinal section of a spinal ganglion and a spinal nerve (cervical
or sacral) : a, Small cells with unmyelinated axons; b, cell of Dogiel's type XI ; c, large cell possessing
a myelinated axon and surrounded by a pericellular plexus; d, bipolar cell.
According to Dogiel every spinal ganglion cell is surrounded by a network of
fine branching and anastomosing fibers; and he believes that these are formed
by the ramifications of fine myelinated and unmyelinated fibers that have
entered the spinal ganglion from the sympathetic nervous system through the
rami communicantes. While the origin of these fibers is open to question, there
can be no doubt that such pericellular networks exist on at least a considerable
proportion of the cells and constitute an important element in the structure of
the ganglion (Fig. 40, c}.
The fiber bundles of the ganglia are composed of both myelinated and un-
5
66 THE NERVOUS SYSTEM
myelinated fibers representing the branches of the axons of the spinal ganglion
cells. Both types of fibers can be followed through the dorsal roots into the
spinal cord, as well as distally into the nerves. In the latter they mingle with
the large myelinated fibers coming from the ventral roots (Fig. 40). When
traced distally in the peripheral nerve the unmyelinated fibers are found to go
in large part to the skin, though a few run in the muscular branches (Ranson,
1911 and 1915).
Classification of the Somatic Afferent Fibers According to Function.
Sherrington (1906) in an instructive book on "The Integra tive Action of the
Nervous System" has furnished us with a useful classification of the elements
belonging to the afferent side of the nervous system. He designates those
carrying impulses from the viscera as interoceptive, and subdivides the somatic
afferent elements into exteroceptive and proprioceptive groups. The extero-
ceptive fibers carry impulses from the surface of the body and from such sense
organs, as the eye and ear, that are designed to receive stimuli from without.
These fibers, therefore, are activated almost exclusively by external stimuli.
The proprioceptive fibers, on the other hand, respond to stimuli arising within
the body itself and convey impulses from the muscles, joints, tendons, and the
semicircular canals of the ear. Each group has receptors or sensory endings
designed to respond to its appropriate set of stimuli, and for each there are
special connections within the brain and spinal cord.
Exteroceptive fibers and sensory endings are activated by changes in the
environment, that is to say, they are stimulated by objects outside the body.
The impulses, produced in this way and carried by these fibers to the spinal
cord, call forth for the most part reactions of the body to its environment;
and, when relayed to the cerebral cortex, they may be accompanied by sensa-
tions of touch, heat, cold, or pain. The receptors are, for the most part, located
in the skin; yet it is convenient to include in the exteroceptive group the pressure
receptors which are closely allied to those for touch, but which lie below the
surface of the body. At this point it should be noted that sensibility to those
forms of contact which include some slight pressure, such as the placing of a
finger on the skin, is not abolished by the section of all of the cutaneous nerves
going to the area in question, since the deeper nerves carry fibers capable of
responding to such contacts (Head, 1905). This deep contact sensibility, which
for lack of a better name we may call "pressure- touch," must not be overlooked
in the analysis of cutaneous sensations.
The balance of evidence is in favor of the assumption that each of the vari-
THE SPINAL NERVES 67
eties of cutaneous sensation is mediated by a separate set of nerve-fibers. But
little progress has as yet been made toward identifying these various func-
tional groups. We know that both myelinated and unmyelinated fibers are
present in the cutaneous nerves (Ranson, 1915), but are not able to say with
certainty which subserve each of the varieties of cutaneous sensation. There
are many good reasons, however, for the belief that painful afferent impulses
and possibly also those of temperature are carried by the unmyelinated fibers,
and that those of the touch and pressure group are mediated by the myelinated
fibers. The evidence on which this statement is based has been briefly sum-
marized on pages 102-104.
Fig. 41. Free nerve endings in the epidermis of a cat's paw: A, Stratum corneum; B, stratum
germinativum Malpighii, and C, its deepest portion; a, large nerve trunk; b, collateral fibers; c,
terminal branches; d, terminations among the epithelial cells. Golgi method. (Cajal.)
All sensory nerve endings in the skin belong to the exteroceptive group,
but it is not so easy to say which ones are responsible for each of the several
varieties of cutaneous sensation, namely, touch, pain, heat, and cold. On
structural grounds we may recognize three principal groups: (1) endings in hair-
follicles, (2) encapsulated nerve endings, and (3) free terminations in the epi-
dermis.
Free Nerve Endings. Some of the myelinated fibers as they approach
their terminations divide repeatedly. At first the branches retain their sheaths,
but after many divisions the myelin sheaths and finally the neurilemma are lost
and only the naked axis-cylinders remain. These enter the epidermis, where,
68
THE NERVOUS SYSTEM
after further divisions, they end among the epithelial cells (Fig. 41). This type
of nerve ending is found in the skin, mucous membranes, and cornea. Similar
endings are also found in the serous membranes and intermuscular connective
tissue.
We do not know what form the endings of the afferent unmyelinated fibers
may take, but it is not unlikely that they also ramify in the epidermis like the
terminal branches of the myelinated fibers just described. It seems certain
that at least a part of the free nerve endings in the epidermis are pain receptors.
In the central part of the cornea, the tympanic membrane, and the dentine
and pulp of the teeth, such free nerve endings alone are present, and pain is the
only sensation that can be appreciated.
Some of the nerve-fibers which enter the epidermis end in disk-like expansions
in contact with specialized epithelial cells (Fig. 42). These have been known
Fig. 42. Merkel's corpuscles or tactile disks from the skin of the pig's snout. The nerve-
fiber, n, branches and each division ends in an expanded disk, m, which is attached to a modified
cell of the epidermis, a; c, an unmodified epithelial cell. (Ranvier, Herrick.)
as Merkel's touch-cells on the supposition that the endings in question are tactile
receptors.
Encapsulated Nerve Endings. Among the encapsulated nerve endings are
the corpuscles of Meissner. These have quite generally been regarded as tactile
end organs and are located in the corium or subepidermal connective tissue of
the hands and feet, forearm, lips, and certain other regions. They are of large
size, oval, possess a thin connective-tissue capsule, and within each terminate
one or more medullated fibers (Fig. 43). Within the capsule the fibers lose their
myelin sheaths, make a variable number of spiral turns, and finally break up
into many varicose branches, which form a complex network. To another
type of encapsulated end organ belong those known as the end bulbs of Krause.
One of these is illustrated in Fig. 44. They are found in the conjunctiva, edge*
of the cornea, lips, and some other localities.
THE SPINAL NERVES
6 9
Fig. 43. Meissner's tactile corpuscle.
Methylene-blue stain. (Dogiel, Bohm-David-
off-Huber.)
Fig. 44. End-bulb of Krause from con-
junctiva of man. Methylene-blue stain.
(Dogiel, Bohm-Davidoff-Huber.)
Fig. 45. Pacinian corpuscles from mesorectum of kitten: A, Showing the fine branches of
the central fiber; B, the network of fine nerve-fibers about the central fiber. Methylene-blue stain.
(Sala, Bohm-Davidoff-Huber.)
The Pacinian corpuscles, two of which are illustrated in Fig. 45, have a very
wide distribution in the deeper parts of the dermis of the hands and feet, in the
7
tendons, intermuscular septa, periosteum, peritoneum, pleura, and pericardium.
They are also numerous in the neighborhood of the joints. According to Her-
rick (1918) it is probable that "by these end organs relatively coarse pressure
may be discriminated and localized (exteroceptive function), and movements
of muscles and joints can be recognized (proprioceptive function)." They are
1 hst
Tt is
Fig. 46. Nerves and nerve endings in the skin and hair-follicles: hst, Stratum corneum; rm,
stratum germinativum Malpighii; c, most superficial nerve-fiber plexus in the cutis; n, cutaneous
nerve; is, inner root sheath of hair; as, outer root sheath; h, the hair itself; dr, glandulae sebaceae.
(Retzius, Barker.)
large oval corpuscles, made up in great part of concentric lamellae of connective
tissue. The axis of the corpuscle is occupied by a core of semifluid substance
containing the termination of a nerve-fiber. The fiber loses its myelin sheath
as it enters the core, through which it passes from end to end. Its terminal
branches end in irregular disks. Side branches are also given off within the core.
THE SPINAL NERVES
Nerve Endings in the Hair-follicles. It has long been known that the hairs
are delicate tactile organs. The hair-clad parts lose much of their responsive-
Fig. 47. Neuromuscular nerve end-organ from a dog. The figure shows the intrafusal
muscle-fibers, the nerve-fibers and their terminations, but not the capsule nor the sheath of Henle.
Methylene-blue stain. (Huber and De Witt.)
ness to touch when the hair is removed. As would be expected on these grounds,
the hair-follicles are richly supplied with nerve endings. Just below the open-
ing of the sebaceous gland into the follicle myelinated nerve-fibers enter it, los-
72 THE NERVOUS SYSTEM
ing their myelin sheaths as they enter. They give off horizontal branches,
which encircle the root of the hair, and from these ascending branches arise
(Fig. 46). Some of these are connected with leaf-like expansions, associated
with cells resembling Merkel's touch-cells.
Practically nothing is known concerning the receptors for sensations of heat
and cold.
Proprioceptive Fibers and Sensory Nerve Endings. To this group belong
the afferent elements which receive and convey the impulses arising in the
muscles, joints, and tendons. Changes in tension of muscles and tendons and
movements of the joints are adequate stimuli for the receptors of this class and
excite nerve impulses which, on reaching the central nervous system, give in-
formation concerning tension of the muscles and the relative position of the
various parts of the body. For the most part, however, these impulses do not
rise into consciousness, but serve for the subconscious control of muscular
activity. The unsteady gait of a tabetic patient illustrates the lack of mus-
cular control that results when these impulses are prevented from reaching the
central nervous system.
The proprioceptive fibers are myelinated and are associated with motor
fibers in the nerves to the muscles. Some follow along the muscles to reach
the tendons. Three types of end organs belong to this group, Pacinian cor-
puscles, muscle spindles, and neurotendinous end organs. Many Pacinian
corpuscles are found in the neighborhood of the joints. They have been de-
scribed in a preceding paragraph.
Neuromuscular End Organs. The afferent fibers to the muscles end on
small, spindle-shaped bundles of specialized muscle-fibers (Fig. 47). These
muscle spindles are invested by connective- tissue capsules; and within each
of them one or more large myelinated nerve-fibers terminate. Within the
spindle the myelin sheath is lost and the branches of the axis-cylinders wind
spirally about the specialized muscle-fibers, or they may end in irregular disks.
Somewhat analogous structures are the neurotendinous end organs or tendon
spindles where myelinated nerve-fibers end in relation to specialized tendon
fasciculi.
CHAPTER VI
THE SPINAL CORD
THE spinal cord, or medulla spinalis, is a cylindric mass of nervous tissue
occupying the vertebral canal. It is 40 to 45 cm. in length, reaching from the
foramen magnum, where it is continuous with the medulla oblongata, to the
level of the first or second lumbar vertebra. Even above this level the vertebral
canal is by no means fully occupied by the cord (Fig. 48), which, as shown in
Fig' 49, is surrounded by protective membranes, while between these and the
wall of the canal is a rather thick cushion of adipose tissue containing a plexus
Extradural fat and venous plexus
Subarachnoid space
Spinal nerve roots
Spinal cord
Dura mater
Ligamentum denticulatum
Fig. 48. Diagram showing the relation of the spinal cord to the vertebral column.
of veins. Immediately surrounding the cord and adherent to it is the delicate,
highly vascular pia mater. This is separated from the thick, fibrous dura mater
by a membrane having the tenuity of a spider web, the arachnoid, which sur-
rounds the subarachnoid space. This space is broken up by subarachnoid
trabeculae and filled with cerebrospinal fluid.
External Form. The spinal cord is not a perfect cylinder, but is somewhat
flattened ventrodorsally, especially in the cervical region. Its diameter is not
uniform throughout, being less in the thoracic than in the cervical and lumbar
portions. That is to say, the cord presents two swellings (Fig. 51). The cer-
vical enlargement (intumescentia cervicalis) comprises all that portion of the cord
73
74
THE NERVOUS SYSTEM
from which the nerves of the brachial plexus arise, that is, the fourth cervical
to the second thoracic segments inclusive. The lumbar enlargement (intumes-
centia lumbalis) is not quite so extensive and corresponds less accurately to the
origin of the nerves innervating the lower extremity. At an early stage in the
embryonic development of the spinal cord these enlargements are not present.
In the time of their first appearance and in their subsequent growth they are
directly related to the development of the limbs.
Below the lumbar enlargement the spinal cord rapidly decreases in size
and has a cone-shaped termination, the conus medullaris, from the end of which
a slender filament, the filum terminale, is prolonged to the posterior surface of
the coccyx (Figs. 50, 51). This terminal filament descends in the middle line,
surrounded by the roots of the lumbar and sacral nerves, to the caudal end of
Septum poslicum
/Posterior spinal artery
_,,,... , , ^^>_- Ligamentum denticulatum
Subarachnoid trabecuke ---.-.
Pia mater -
Epidural trabeculce *=;2?5
Anterior spinal artery'
--Dura mater
*- Subdural space
Arachnoid
'-Nerve root
Subarachnoid cavity
Linca splendens
Fig. 49. Diagram of the spinal cord and meninges.
the dural sac at the level of the second sacral vertebra. Here it perforates the
dura mater, from which it receives an investment, and then continues to the
posterior surface of the coccyx. The last portion of the filament with its dural
investment is often called the filum of the spinal dura mater (filum durae matris
spinalis). The filum terminale is composed chiefly of pia mater; but in its
rostral part it contains a prolongation of the central canal of the cord.
The spinal cord shows an obscure segmentation, in that It gives origin to
thirty-one pairs of metameric nerves. These segments may be somewhat
arbitrarily marked off from each other by passing imaginary planes through the
highest root filaments of each successive spinal nerve (Donaldson and Davis,
1903). The highest of these planes, being just above the origin of the first cer-
vical nerve, marks the separation of the spinal cord from the medulla oblongata.
THE SPINAL CORD
75
This is again an arbitrary line of separation, since both as to external form
and internal structure the cord passes over into the medulla oblongata by in-
Medulla oblongala
f- N. cenicalis VIII
' Ventral root of N.
T.I II
Dorsal root of N.
T.IV
Lateral funiculus
Spinal dura mater
- N. thoricalis XII
\- Cauda equina
I- N. lumbalis V
Filum of spinal dura
mater
.Medulla oblongata. _
I Anterior median fissure
A nterolateral sulcus
-Cervical enlargement
-A nterior funiculus
-Thoracic portion of- -
spinal cord
I Lumbar enlargement
Conus medullaris
Filum terminale
Rhomboid fossa
Posterior median
sulcus
Posterior funic-
ulus
Posterior inter-
mediate sulcus
Dorsal root
- Spinal nerve
.
^-Cauda equina
Fig. 50. Fig. 51. Fig. 52.
Figs. 50-52. Three views of the spinal cord and rhombencephalon : Fig. 50, Lateral view
with spinal nerves attached; Fig. 51, ventral view with spinal nerves removed; Fig. 52, dorsal
view with spinal nerves attached. (Modified from Spalteholz.)
sensible gradations. According to this method of subdivision there are in the
cervical portion of the cord eight segments, in the thoracic twelve, in the lumbar
five, and in the sacral five, while there is but one coccygeal segment.
76 THE NERVOUS SYSTEM
Several longitudinal furrows are seen upon the surface of the cord (Figs. 51,
52). Along the middle line of the ventral surface is the deep anterior median
fissure (fissura mediana anterior). This extends into the cord to a depth
amounting to nearly one-third of its anteroposterior diameter and contains a
fold of pia mater. Along the middle line of the dorsal surface there is a shallow
groove, the posterior median sulcus (sulcus medianus posterior). As may be
seen in cross-sections of the spinal cord, it is divided into approximately sym-
metric lateral halves by the two furrows just described and by the posterior
median septum (Figs. 55, 56, 57). On either side, corresponding to the line of
origin of the ventral roots, is a broad, shallow, almost invisible groove, the
anterolateral sulcus (sulcus lateralis anterior). And again on either side, cor-
responding to the line of origin of the dorsal roots, is the narrower but deeper
posterolateral sulcus (sulcus lateralis posterior). These six furrows extend the
entire length of the spinal cord. In the cervical region an additional longi-
tudinal groove may be seen on the dorsal surface between the posterior median
and posterolateral sulci, but somewhat nearer the former. It is known as the
posterior intermediate sulcus and extends into the thoracic cord, where it grad-
ually disappears.
Funiculi. By means of these furrows and the subjacent gray matter each
lateral half of the cord is subdivided into columns of longitudinally coursing
nerve-fibers known as the anterior, lateral, and posterior funiculi (funiculus
anterior, funiculus lateralis et funiculus posterior). In the cervical and upper
thoracic regions the posterior intermediate sulcus divides the posterior funiculus
into a medial portion, the fasciculus gracilis, and a lateral portion, the fasciculus
cuneatus.
Nerve Roots. From the lateral funiculus in the upper four to six cervical
segments there emerge, a little in front of the dorsal roots of the spinal nerves,
a series of root filaments which unite to form the spinal root of the accessory
nerve (Fig. 125). This small nerve trunk ascends along the side of the cord,
enters the cranial cavity through the foramen magnum, and carries to the
accessory nerve the fibers for the innervation of the sternocleidomastoid and
trapezius muscles.
From the posterolateral sulcus throughout the entire length of the spinal
cord emerge an almost uninterrupted series of root filaments (fila radicularia).
Those from a given segment of the cord unite to form the. dorsal root of the cor-
responding spinal nerve. The filaments of the ventral roots emerge from the
broad, indistinct anterolateral sulcus in groups, several appearing side by side,
THE SPINAL CORD
77
rather than in the accurate linear order characteristic of the dorsal roots. Those
from a given segment unite with each other to form a ventral root; and that in
turn joins with the corresponding dorsal root just beyond the spinal ganglion to
form the mixed nerve (Fig. 50).
Relation of the Spinal Cord and Nerve Roots to the Vertebral Column.
At an early fetal stage the spinal cord occupies the entire length of the vertebral
Infrahyoid muscles
Diaphragm
Muscles of shoulder, arm,
and hand
Cervical segments of spinal cord
Thoracic segments of spinal cord
Lumbar segments of spinal cord
Sacral and coccygeal segments of
spinal cord
Abdominal musdes
Flexors of hip /
Extensors of the kneel
and adductors of hip[
Other muscles of thigh,
leg, and foot
Perinea! and anal mus-
cles
Fig. 53. Diagram showing the level of the various segments of the spinal cord with reference to
the vertebra?, with a table showing the distribution of the fibers of the several ventral roots.
canal and the spinal nerves pass horizontally lateralward to their exit through
the intervertebral foramina. As development progresses the vertebral column
increases in length more rapidly than the spinal cord, which, being firmly an-
chored above by its attachment to the brain, is drawn upward along the canal,
until in the adult it ends at about the lower border of the first lumbar vertebra.
78 THE NERVOUS SYSTEM
At the same time the roots of the lumbar and sacral nerves become greatly
elongated. They run in a caudal direction from their origin to the same inter-
vertebral foramina through which they made their exit before the cord shifted
its position. Since the thoracic portion of the cord has changed its relative
position but little, and the cervical part even less, the cervical roots run almost
directly lateralward, while those of the thoracic nerves incline but little in a
caudal direction.
Since the spinal cord ends opposite the first or second lumbar vertebra, the
roots of the lumbar, sacral, and coccygeal nerves, in order to reach their proper
intervertebral foramina, descend vertically in the canal around the conus medul-
laris and filum terminale. In this way there is formed a large bundle, which is
composed of the roots of all the spinal nerves below the first lumbar and has
been given the very descriptive name cauda equina.
The amount of relative shortening of the various segments of the cord differs
in different individuals. In Fig. 53, where the quadrilateral areas represent the
bodies of the vertebrae, we have indicated the average position of each segment
of the spinal cord. This figure is based on data published by Reid (1889). It is
obvious that the segments are longer in the thoracic than in the cervical and
lumbar portions of the cord, while the sacral segments are even shorter (see
also Fig. 59).
We have been at some pains to explain the development of the cauda equina
and the vertebral level of the various segments of the spinal cord because these
are matters of much practical importance. In spinal puncture the needle is
made to enter the subdural space caudal to the termination of the cord. In
locating lesions of the spinal cord it is necessary to know the position of its
various segments with reference to the vertebras. It is particularly important
to be able to distinguish between an injury to the lower part of the spinal cord
and one which involves only the nerve roots in the cauda equina, since, although
the symptoms in the two cases may be nearly identical, damage to the spinal
cord is irreparable, while the nerve roots will regenerate.
The Spinal Cord in Section. When a section is made through any part of
the brain or spinal cord one sees at once that they are composed of two kinds
of tissue the one whitish in color, the other gray, tinged with pink. The white
substance consists chiefly of myelinated fibers, the gray is made up of nerve-
cells, dendrites, unmyelinated and myelinated fibers, and many blood-vessels.
Both have a supporting framework of neuroglia.
The gray substance (substantia grisea) of the spinal cord is centrally placed
THE SPINAL CORD
79
and forms a continuous fluted column, which is everywhere enclosed by the
white matter (Fig. 54). In cross-section it has the form of a letter H (Fig. 55).
There is a comma-shaped gray field in each lateral
half of the cord, and these are united across the
middle line by a transverse gray bar. The enlarged
anterior end of the comma has been known as the ven-
tral horn, the tapering posterior end as the dorsal
horn, and the transverse bar as the gray commissure.
But, when it is remembered that the gray substance
forms a continuous mass throughout the length of the
spinal cord, it will be seen that the term "column"
is more appropriate than "horn." The long gray mass
in either lateral half of the cord is convex medially and concave laterally. It
projects in a dorsolateral direction as the posterior column (columna posterior).
As seen in a cross-section of the cervical cord, the posterior column is rela-
tively long and narrow and nearly reaches the dorsolateral sulcus (Fig. 55).
Fig. 54. Diagram of gray
columns of spinal cord.
Posterior intermediate sulcus and septum
Collaterals from cuneate fasc.
Substantia gelaiinosa
Posterolateral sulcus
\
Ceraix .
Posterior column{ ^ if?" ~ \
Posterior median sulcus and septum
Fasciculus gracilis \ Posterior
\ Fasckulus cuneatus j fnniculus
Dorsal root
Dorsolateral fasciculus
(Lissauer)
lateral funiculus
Reticular formation
Posterior ...|
com.
Anterior ._S
gray com.
Anterior -
white com.
Anterior'
column
Anterolateral sulcus ' .7'^^ _ -Anterior funiculus
A nterior median fissure
Fig. 55. Section through seventh cervical segment of the spinal cord of a child. Pal-Weigert
method.
It presents a constricted portion known as the cervix, a pointed dorsal extrem-
ity or apex, and between the two an expanded part sometimes called the caput.
The apex consists largely of a special variety of gray substance, gelatinous in
80 THE NERVOUS SYSTEM
appearance in the fresh condition and very difficult to stain by neurologic meth-
ods, which in sections has a A -shaped outline. It is known as the substantia
gelatinosa Rolandi. In the thoracic portion the posterior column, which is here
very slender, does not come so close to the surface; and in the lumbosacral seg-
ments it is much thicker (Figs. 56, 57).
The anterior column is relatively short and thick and projects toward the
anterolateral sulcus. It contains the cells of origin of the fibers of the ventral
root. From its lateral aspect nearly opposite the gray commissure there pro-
jects a triangular mass, known as the lateral column (columna lateralis). This
is prominent in the thoracic and upper cervical segments; but it blends with
the expanded anterior column in the cervical and lumbar enlargements (Fig. 56).
Posterior median sulcus and septum Posterior funiculus
Substantia gelatinosa \ Dorsolateral fasciculus (Lissauer)
Posterolateral sukus ^^^^^ Dorsal root
BiBbU; Lateral funiculus
Apex oj posterior column
Nucleus dorsalis
lateral column
'! ^---^-- ^Central canal
Anterior white commissure ' ^iwKSB^H ^f^^r ....
. . , .--' ----. y-^ Anterior funicmus
Anterior column' ;~^--^_ . ,. ,.
" Anterior median jissure
Fig. 56. Section through the seventh thoracic segment of the spinal cord of a child. Pal-Weigert
method.
The reticular formation (formatio reticularis) , situated just lateral to the cer-
vix of the posterior column in the cervical region, is a mixture of gray and
white matter (Fig. 55). Here a network of gray matter extends into the white
substance, breaking it up into fine bundles of longitudinal fibers. The reticular
formation is most evident in the cervical region, but traces of it appear at other
levels.
The gray commissure contains the central canal, and by it is divided into the
posterior commissure (commissura posterior) and the anterior gray commissure
(commissura anterior grisea). Ventral to the latter many medullated fibers
cross the midline, constituting the anterior white commissure.
The cavity of the neural tube persists as the central canal, which lies in the
gray commissure throughout the entire length of the cord. The canal is so
small as to be barely visible to the naked eye. It is lined with ependymal
THE SPINAL CORD
8l
epithelium and the lumen is often blocked with epithelial debris. The canal,
which is narrowest in the thoracic region, expands within the lower part of the
conus medullaris to form a fusiform dilatation, the ventriculus terminates.
Posterior median sulcus and septum
Collaterals from fasciculus cuneat-us \ Posterior fun i en I us
Dorsal root
I
Dorsolateral fascicuhis (Lissauer}
! Poster olateral sulcus
Substantia gelati
{4 "bex -
r
Lervix
Posterior commissure
Anterior gray -
commissure
Anterior white com...-
Anterior column.*-
Fig. 57. Section through the fifth lumbar segment of the spinal cord of a child. Pal-Weigert
method.
Dorsal roots of lumbar and sacral nerves
Posterior fun iculus
.,
: ''-'- Substantia gelatinosa
Dorsolateral fasciculus
&5HB^. Posterior column
^- Lateral funiculus
'Anterior column
*~ Ventral roots oj "lumbar and
sacral nerves
Fig. 58. Section of the third sacral segment of the human spinal cord and the lumbosacral nerve
roots of the cauda equina. Pal-Weigert method.
The White Substance. The long myelinated fibers of the cord, arranged in
parallel longitudinal bundles, constitute the white substance which forms a
82
THE NERVOUS SYSTEM
thick mantle surrounding the gray columns. In each lateral half of the cord it
is divided into the three great strands or funiculi, which have been described
White matter.
Grey matter.
-Ertrire secMon.
1OO
o
eo
40
20
I II lfl IY
WIYIO 1 H HI W Y H MI H1I IX X XI XII 1 II IfllVYI
Fig. 59. Curves showing the variations in sectional area of the gray matter, the white matter, and
the entire cord in the various segments of the human spinal cord. (Donaldson and Davis.)
on the surface of the cord. The anterior funiculus (funiculus anterior) is bounded
by the anterior median fissure, the anterior column, and the emergent fibers
of the ventral roots. The lateral funiculus (funiculus lateralis) lies lateral to
V//C- VII I C
vine -ID
II D
VII D
XII D
III S
IV 5 C
Fig. 60. Outline drawings of sections through representative segments of the human spinal cord.
the gray substance between the anterolateral and posterolateral sulci, i. e.,
between the lines of exit of the ventral and dorsal roots. The posterior funiculus
(funiculus posterior) is bounded by the posterolateral sulcus and posterior col-
THE SPINAL CORD 83
CHARACTERISTIC FEATURES OF TRANSVERSE SECTIONS AT VARIOUS LEVELS OF THE SPINAL CORD
Level.
Cervical.
Thoracic.
Lumbar.
Sacral.
Outline
Oval, greatest di-
ameter transverse
Oval to circular
Nearly circular
Circular to
quadrilateral
Volume of gray
matter
Large
Small
Large
Relatively
large
Anterior gray
column
Massive
Slender
Massive
Massive
Posterior gray
column
Relatively slender,
but extends far
posteriorly
Slender
Massive
Massive
Lateral gray
column
Absorbed in the
anterior except in
the upper three
cervical segments
\Vell marked
Absorbed in the
anterior column
Present
Processus
reticularis
Well developed
Poorly developed
Absent
Absent
White matter
In large amount
Less than in the
cervical region,
but relatively a
large amount in
comparison to the
gray matter
Slightly less than in
the thoracic re-
gion; very little
in comparison to
the large volume
of the gray
Very little
Sulcus interme-
dius posterior
Present throughout
Present in upper
seven thoracic
segments
Absent
Absent
umn on the one side, and the posterior median septum on the other. The sep-
tum, just mentioned, completely separates the two posterior funiculi from each
other. Incomplete septa project into the white substance from the enveloping
pia mater. One of these, more regular than the others, enters along the line of
the posterior intermediate sulcus. It is restricted to the cervical and upper
thoracic segments, is known as the posterior intermediate septum, and divides
the posterior funiculus into two bundles, the more medial of which is known
as the fasciculus gracilis, while the other is called the fasciculus cuneatus.
Characteristics of the Several Regions of the Spinal Cord. It will be ap-
parent from Figs. 55-58 that the size and shape of the spinal cord, as seen in
transverse section, varies greatly at the different levels and that the relative
proportion of gray and white matter is equally variable. Two factors are
84 THE NERVOUS SYSTEM
primarily responsible for these differences. One of these is the variation in the
size of the nerve roots at the different levels; for where great numbers of nerve-
fibers enter, they cause an increase in the size of the cord and particularly in
the volume of the gray matter. It has already been pointed out that the cer-
vical and lumbar enlargements are directly related to the large nerves supply-
ing the extremities. The second factor is this: Since all levels of the cord are
associated with the brain by bundles of long fibers, it is obvious that such long
fibers must increase in number and the white matter increase in volume as we
follow the cord from its caudal end toward the brain. All this is well illus-
trated in a diagram by Donaldson and Davis reproduced in Fig. 59.
The outline of a section of the spinal cord at the fourth sacral segment is some-
what quadrilateral. The total area is small and the greater part is occupied
by the thick gray columns (Fig. 60). The size of the cord is much greater at
the level of the first sacral and fifth lumbar segments, as might be expected from
the large size of the associated nerves (Figs. 57, 60). There is both an absolute
and a relative increase in the white substance, which here contains the long
paths connecting the sacral portions of the spinal cord with the brain. Both
the anterior and posterior columns are massive, and the anterior presents a
prominent lateral angle. The large nerve-cells in the lateral part of the an-
terior column give rise to the fibers which run to the muscles of the leg. At the
level of the seventh thoracic segment (Figs. 56, 60) the cross-sectional area is less
than in the lumbar enlargement. Corresponding to the small size of the tho-
racic nerves the gray matter in this region is much reduced, both anterior and
posterior columns being very slender. The apex of the latter is some distance
from the surface and its cervix is thickened by a column of cells known as the
nucleus dorsalis. The columna lateralis is prominent. The white matter is
somewhat more abundant than in the lumbar region, and increases slightly in
amount as we follow the cord rostrally through the thoracic region (Fig. 59).
A transverse section at the level of the seventh cervical segment is elliptic in
outline and has an area greater than that of any other level of the cord (Figs.
55, 60). The white matter is voluminous and contains the long fiber tracts
connecting the brain with the more caudal portions of the cord. The gray
matter is also abundant, as we might expect from the large size of the seventh
cervical nerve. The ventral column is especially thick and presents a prominent
lateral angle. The large laterally placed nerve-cells of the anterior column are
associated with the innervation of the musculature of the arm. The posterior
column is relatively slender, but reaches nearly to the dorsolateral sulcus.
THE SPINAL CORD 85
MICROSCOPIC ANATOMY
Neuroglia. Occupying the interstices among the true nervous elements of
the central nervous system is a peculiar supporting tissue, the neuroglia, which
is of ectodermal origin. In the chapter on Histogenesis we learned that from
the original epithelium of the neural tube there are differentiated spongioblasts
and neuroblasts, as well as a special epithelial lining for the tube, the ependyma.
Fig. 61. Ependyma and neuroglia in the region of the central canal of a child's spinal cord:
A, Ependymal cells; B and D, spider cells in the white and gray matter, respectively; C, mossy
cells. Golgi method. (Cajal.)
The latter consists of long nucleated columnar cells which line the central canal
of the spinal cord as well as the ventricles of the brain (Fig. 61). In fetal life
their free ends bear cilia, which project into the lumen of the tube, and fine
processes from the outer ends extend to the periphery of the cord. In the adult
there are no cilia and the peripheral processes reach the surface only along the
posterior median septum and in the anterior median fissure.
86 THE NERVOUS SYSTEM
The neuroglia cells are differentiated from the spongioblasts. These, when
stained by the Golgi method, appear as small cells with many processes. Some
have long slender processes, the spider cells or long rayed astrocytes; others have
short thick varicose processes, the mossy cells or short rayed astrocytes (Fig.
61). Special neuroglia stains, like that of Weigert, show that an astrocyte is
composed of a glia cell associated with many glia fibers. Some authors main-
tain that the fibers run through the cytoplasm, while others assert that they
merely pass along the surface of the cell. In any case the fibers are to be re-
garded as products of these cells. Neuroglia cells and fibers are found every-
where throughout the gray and white matter of the spinal cord, forming a sup-
porting framework for the nervous elements. A special condensation of neu-
Unmyelinated fibers
inated fibers
Fig. 62. From a cross-section through the spinal cord of a rabbit showing the structure of the white
matter as revealed by the Cajal method. (Cajal.)
roglia surrounds the central canal and is known as the substantia gelatinosa
centralis. In addition to the neuroglia this contains some nerve-fibers and
cells. Beneath the pia mater and closely investing the spinal cord externally
is a thin stratum of neuroglia, the glial sheath, which dips into the cord along
with the pial septa. The posterior median septum is composed of neuroglia
and greatly elongated ependymal elements, and is in no part formed by the
pia mater.
White Substance. The white matter of the spinal cord consists of longi-
tudinally coursing bundles of nerve-fibers, bound together by a feltwork of
neuroglia fibers in which are scattered neuroglia cells. A majority of the neu-
roglia fibers run in a direction transverse to the long axis of the nerve-fibers.
Blood-vessels enter the cord from the pia mater and are accompanied by con-
t THE SPINAL CORD 87
nective tissue from the pia and by the subpial neuroglia. It has been generally
supposed that the white fascicles of the cord were composed almost exclusively
of myelinated fibers; and it is true that these, partly because of their size, are
the most conspicuous elements. In cross-sections stained by the Weigert
method the myelin sheaths alone are stained; and since the fibers are cut at
right angles to their long axes, they appear as rings. Cajal (1909) has shown
that there are also great numbers of unmyelinated fibers in the longitudinal
fascicles of the cord (Fig. 62). The different fascicles differ not only in the size
of their myelinated fibers but also in the proportion of unmyelinated fibers
which they contain. The fasciculus dorsolateralis or tract of Lissauer (Fig. 63)
contains fine myelinated fibers with great numbers of unmyelinated axons.
Fig. 63. From a cross-section of the spinal cord of the cat; a narrow strip extending across
the apex of the posterior gray column: a, Fasciculus cuneatus; b, fasciculus dorsolateralis (Lis-
sauer) ; c, dorsal spinocerebellar tract. The unmyelinated fibers appear as black dots. Pyridin-
silver method.
Close to it lies the dorsal spinocerebellar tract which is composed almost ex-
clusively of large myelinated fibers.
Gray Substance. The gray matter is composed of nerve-cells, including
their dendrites, and of unmyelinated axons and smaller numbers of myelinated
fibers all supported by a neuroglia framework and richly supplied with capil-
lary blood-vessels. The axons of the cells of Golgi's Type I are very long and
run out into the white substance or into the ventral roots. Those of the cells
of his Type II are short and end within the gray matter. In addition, great
numbers of collaterals from the dorsal root fibers and from the longitudinal
fibers of the cord, as well as terminal branches of these fibers, enter the gray
substance and ramify extensively within it, entering into synaptic relations
with the neurons which it contains. The branches of the myelinated fibers
soon lose their sheaths, and it is this relative scarcity of myelin which gives to
88
THE NERVOUS SYSTEM
this substance its gray appearance. The ramification of dendrites and unmy-
elinated fibers forms a very intricate feltwork throughout the gray substance
(Fig: 64).
The nerve-cells of the spinal cord vary greatly in size. The largest are
situated in the anterior column and may measure more than 100 micra. They
are all multipolar, possess each a single axon, and may be classified in four groups:
(1) Some of the cells, found in the posterior horn and particularly in the sub-
stantia gelatinosa Rolandi, belong to Golgi's Type II, with short axons confined
to the gray substance. These, however, are present in relatively small numbers
in the spinal cord. (2) The motor cells, situated in the anterior column and
Fig. 64. From a section through the spinal cord of a monkey; showing part of the an-
terior gray column including a multipolar nerve-cell and the surrounding neuropil. Pyridin-
silver method.
most numerous in the cervical and lumbar enlargements, are of large size and
possess axons which leave the cord in the ventral roots. (3) Smaller cells are
present in the lateral column in the thoracic region and give rise to the visceral
efferent fibers of the ventral roots (Fig. 37). (4) Other cells of small or medium
size, found chiefly in the posterior column, possess axons which pass into the
white matter, where they bend sharply to become ascending or descending
fibers, or divide dichotomously into ascending and descending branches (Fig.
68). Some of the ascending fibers reach the brain; the others merely connect
the different levels of the spinal cord. The fibers of the latter group constitute
the fasciculi proprii and vary greatly in length, some connecting adjacent,
THE SPINAL CORD 89
others, more remote, segments. Their collateral and terminal branches re-
enter and ramify within the gray substance. Those which remain throughout
in the same lateral half of the cord are called association fibers; while others,
known as commissural fibers, cross the median plane chiefly in the white com-
missure (Fig. 68). Some of the commissural fibers are short and confined to a
single level of the cord (Fig. 66).
Cell-columns. The nerve-cells are not uniformly distributed throughout
the gray matter, for many of them are arranged in longitudinal cell-columns.
In transverse sections each of these columns appears as a distinct group of
cells, somewhat separated from other similar groups within the gray matter
(Fig. 65). The large motor cells of the anterior column, which give origin to
the ventral root fibers, form several subgroups. One of these, known as the
anteromedian cell-column, occupies the medial part of the anterior column through-
out almost its entire length, being absent only in the fifth lumbar and first
sacral segments. Behind it is the posteromedian cell-column, which is, however,
present only in the thoracic and first lumbar segments and for a short stretch
in the cervical region. The axons from these two medial groups of cells prob-
ably supply the musculature of the trunk. In the cervical and lumbar enlarge-
ments there are laterally placed groups of cells the axons of which supply the
muscles of the limbs. These are: (1) the anterolateral cell-column, present in
the fourth to the eighth cervical and in the second lumbar to the second
sacral segments; (2) the posterolateral cell-column in the last five cervical,
last four lumbar, and first three sacral segments; (3) the retro posterolateral
cell-column in the eighth cervical, first thoracic, and first three sacral seg-
ments, and (4) the central cell-column in the second lumbar to the second sacral
segments.
The intermediolateral cell-column is found in the lateral column in the tho-
racic region of the cord and is prolonged downward into the upper lumbar seg-
ments. It is composed of small cells, the axons of which run through the ven-
tral roots, spinal nerves, and white rami communicantes into the sympathetic
nervous system (Fig. 37). They have to do with the innervation of smooth
and cardiac muscle and glandular tissue. The longitudinal extent of this
column corresponds quite accurately to that of the spinal origin of the white
rami. A group of cells, having a similar function, is also found in the third
and fourth sacral segments.
The cells of the posterior gray column are smaller, as a rule, than those of the
ventral column: and except for the nucleus dorsalis they are not arranged in
9 o
THE NERVOUS SYSTEM
definite groups. They are concerned with the reception and distribution of
the impulses entering along the fibers of the dorsal roots.
Si
S4-
Fig. 65. Outline sketches of ventral horn of left side of cord at different levels, showing the
relative number and position of the chief cell-groups: C\, C\, T 6 , etc., indicate the segments e. g.,
first cervical, fourth cervical, sixth thoracic; C (b), lower part of eighth cervical. The following
letters designate the cell-groups: v-m, Anteromedian; d-nt, posteromedian ; v-l, anterolateral ;
d-l, posterolateral ; p. d-l, retroposterolateral; v in LV, L*, ventral; c in L%, L 4 , S\, central; /. c. in
T 6 , Ti2, intermediolateral ; ace. in C\, C 4 , accessorius; phr. in C 4 , phrenic; Cl.c. in T 6 , Tu, nucleus
dorsalis. (Bruce, Quain's Anatomy.)
The nucleus dorsalis, or column of Clarke, is a group of large cells in the
medial part of the base of the posterior column. It extends from the last cer-
THE SPINAL CORD g r
vical or first thoracic to the second or third lumbar segments. It is a prom-
inent feature in cross-sections of the thoracic cord, appearing as a well-defined
oval area richly supplied with collaterals from the dorsal roots. The cells have
an oval or pyriform shape; each has several dendritic processes and an axon
which enters the lateral funiculus, within which it runs toward the cerebellum
in the dorsal spinocerebellar tract.
The Spinal Reflex Mechanism. In the next chapter we will consider at
length the long ascending and descending paths in the white substance of the
cord by which afferent impulses from the spinal nerves reach the brain, and
those through which the motor centers of the brain exert in return a controlling
inf uence over the spinal motor apparatus. But fully as important as these are
the purely intraspinal connections the spinal reflex mechanism.
Fig. 66. Diagrammatic section through the spinal cord and a spinal nerve to illustrate a
simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron;
/, commissural neuron.
A reflex arc in its simplest form may be made up of only two neurons, the
primary sensory and motor neurons with a synapse in the gray matter of the
anterior column (Fig. 66). It consists of the following parts: (1) a receptor,
the peripheral sensory endings; (2) a conductor, the afferent nerve-fiber; (3) a
center, including the synapse in the anterior column; (4) a second conductor,
the efferent nerve-fiber, and (5) an effector, the muscle-fiber. Usually, how-
ever, there are interposed between the primary sensory and motor elements
one or more intermediate neurons. These, when restricted to one side of the
cord, are known as association neurons; when their axons cross the median
plane, as many of them do through the anterior white commissure, they are
called commissural neurons. When the circuit is complete within a single neural
92 THE NERVOUS SYSTEM
segment it may be said to be intrasegmental (Fig. 66) ; if it extends through two
or more such segments it is an intersegmental reflex arc.
Intersegmental Reflex Arcs. Impulses entering the spinal cord through a
given dorsal root may be transmitted to the primary motor neurons of another
segment in one of two ways: (1) by way of the ascending and descending branches
of the dorsal root fibers, and (2) along the fibers of the fasciculi proprii (Fig. 67) .
A full account of these two pathways will be presented in the next chapter,
but a word of explanation is required here. The fibers of the dorsal root divide,
Fig. 67. Diagram of the spinal cord, showing the elements concerned in a diffuse unilat-
eral reflex: a, Spinal ganglion cell; b, motor cell in anterior column; c, association neuron.
(Cajal.)
soon after their entrance into the cord, into long ascending and shorter descend-
ing branches, which together form the greater part of the posterior funiculus
and give off many collaterals to the gray matter of the successive levels of the
cord (Fig. 67). Many of the ascending branches reach the brain; but the others
terminate, as do the descending branches and all the collaterals, in the gray
matter of the cord (Fig. 68). The fasciculi proprii immediately surround the
gray columns (Fig. 68) and consist of ascending and descending fibers, which
arise and terminate within the gray substance of the cord. Most of these
fibers remain on the same side as association fibers concerned in unilateral re-
THE SPINAL CORD
93
flexes. Others cross in the anterior white commissure and are commissural
fibers concerned in crossed reflexes. Afferent impulses may be transmitted
along the cord in either direction by the branches of the dorsal root fibers; or by
means of synapses in the gray matter they may be transferred to the long asso-
ciation and commissural fibers and conveyed to the primary motor neurons of
the same or opposite side in more or less distant segments. The course of a
nerve impulse in a unilateral intersegmental reflex is indicated on the left side
Dorsal root
Ventral root
Ascending branch of dorsal root fiber
Association fibers -'-''
Descending branch of dorsal foot fiber
Dorsal root
Commissural fibers
I
Ventral root
Fig. 68. Diagram of the spinal cord, showing the elements concerned in intersegmental reflexes.
of Fig. 68, while on the right side of the same figure are shown the elements
concerned in crossed reflexes.
The observations of Coghill (1913 and 1914) and of Herrick and Coghill (1915) tend to
show that the simple form of reflex arc illustrated in Fig. 66 is not the primitive type. In
larval Amblystoma the first arcs to become functionally mature are composed of chains
of many neurons, so arranged that every cutaneous stimulus elicits the same complex response
of the entire somatic musculature, i. e., the swimming movement. It is of particular interest
to note that in this primitive reflex mechanism the sensory fibers arise from giant cells located
within the spinal cord and that the ventral root fibers are collaterals from the central motor
tract. In adult Amblystoma these sensory and motor elements are replaced by the usual
type of primary sensory and motor neurons.
94
THE NERVOUS SYSTEM
We may mention as an example of a reflex arc involving many segments of
the cord the ''scratch-reflex" of the dog, which has been very carefully investi-
gated by Sherrington (1906). If, some time after transection of the spinal cord
in the low cervical region, the skin covering the dorsal aspect of the thorax be
stimulated by pulling lightly on a hair, the hind limb of the corresponding side
begins a series of rhythmic scratching movements. By degeneration experi-
ments it was shown that this reflex arc probably includes the following elements:
(1) a primary sensory neuron from the skin to the spinal gray matter of the
corresponding neural segment; (2) a long descending association neuron from the
Fig. 69. Diagram of the spinal arcs involved in the scratch-reflex: Ra and Rp, Receptive
paths from hairs in the dorsal skin of left side; Pa and P/3, association neurons; FC, motor fibers of
ventral root. (Sherrington.)
shoulder to the leg segments, and (3) a primary motor neuron to a flexor muscle
of the leg (Fig. 69).
A primary motor neuron seldom, if ever, belongs exclusively to one arc, but
serves as the final channel to which many streams converge. Its perikaryon
gives off wide-spread dendritic processes, through which it comes into relation
with the ramifications of axons from many different sources. In this way
impulses reach it from the dorsal roots, and from the fasciculi proprii of the
spinal cord, as well as from a number of tracts which descend into the spinal
cord from centers in the brain (the corticospinal, rubrospinal, tectospinal, and
vestibulospinal tracts). The primary motor neuron is, as Sherrington has said,
"the final common path."
CHAPTER VII
FIBER TRACTS OF THE SPINAL CORD
THE fibers composing the white substance of the spinal cord are not scat-
tered and intermingled at random, but, on the contrary, those of a given func-
tion are grouped together in more or less definite bundles. A bundle of fibers
all of which have the same origin, termination, and function is known as a fiber
tract. The funiculi of the spinal cord are composed of many such tracts of
longitudinal fibers, which, while occupying fairly definite areas, blend more or
less with each other, in the sense that there is considerable intermingling of the
fibers of adjacent tracts. It is convenient to have a name for certain obvious
subdivisions of the funiculi which contain fibers belonging to more than one tract.
Such a mixed bundle is properly called a. fasciculus.
THE INTRAMEDULLARY COURSE OF THE DORSAL ROOT FIBERS
The central end of a dorsal root breaks up into many rootlets or filaments
(fila radicularia) , which enter the spinal cord in linear order along the line of
the posterior lateral sulcus. As it enters the cord each filament can be seen to
separate into a larger medial and a much smaller lateral division. The fibers of
the medial division are of relatively large caliber and run over the tip of the
posterior column into the posterior funiculus (Fig. 72). Those of the lateral
division are fine and enter a small fascicle which lies along the apex of the pos-
terior column, the fasciculus dorsolateralis or tract of Lissauer. Very soon
after their entrance into the cord each dorsal root fiber divides in the manner of
a Y into a longer ascending and a shorter descending branch (Fig. 70).
The ascending branches of the fibers of the medial division of the dorsal root
run for considerable but varying distances in the posterior funiculus; some from
each root reach the medulla oblongata, others terminate at different levels in the
gray matter of the spinal cord. At the level of their entry into the cord these
fibers occupy the lateral portion of the fasiculus cuneatus; but in their course
cephalad, as each successive root adds its quota, those from the more caudal
roots are displaced medianward. In this way the longer fibers come to occupy
the medial portion of the posterior funiculus (Fig. 71). In the cervical regior
95
96 THE NERVOUS SYSTEM
the long ascending fibers from the sacral, lumbar, and lower thoracic roots
constitute a well-defined medially placed bundle, the fasciculus gracilis, sepa-
rated from the rest of the posterior funiculus by the posterior intermediate
septum. Those of the long ascending fibers, which finally reach the brain,
terminate in gray masses in the posterior funiculi of the medulla oblongata
Fig. 70. Bifurcation of the dorsal root fibers within the spinal cord into ascending and
descending branches, which in turn give off collaterals; the termination of some of these col-
laterals in synaptic relation to cells of the posterior gray column. (Cajal, Edinger.)
(nucleus of the funiculus gracilis and nucleus of the funiculus cuneatus). Since
the number of these long ascending branches must increase from below upward
it is easy to understand the progressive increase in size of the posterior funiculus
from the sacral to the cervical region (Fig. 60).
The fasciculus gracilis and fasciculus cuneatus are composed for the most
FIBER TRACTS OF THE SPINAL CORD
97
Fasc. gracilis
\ Fasc. cuneatus
part of these ascending branches of the dorsal root fibers, the former contain-
ing those which have the longest intramedullary course.
The descending branches of the fibers of the medial division of the dorsal
root are all relatively short. The shortest terminate at once in the gray matter
of the posterior column. Others descend in the fasciculus interfascicularis, or
comma tract of Schultze, which is situated near the center of the posterior fu-
niculus; and still others run near the posterior median septum in the septomar-
ginal fasciculus (Fig. 76). In both of these fas-
cicles they are intermingled with descending fibers,
arising from cells within the gray matter of the spinal
cord.
Collaterals. At intervals along both ascending
and descending branches collaterals are given off which
run ventrally to end in the gray matter (Fig. 70).
They are much finer than the fibers from which they
arise, and the total number arising from a given fiber
is rather large. Some of them end in the ventral
gray column; others, in the posterior gray column,
including the substantia gelatinosa and the nucleus
dorsalis; still others run through the dorsal com-
missure to the opposite side of the cord, where they
appear to end in the posterior columns (Fig. 72). In
Fig. 70 there are illustrated the arborizations formed
by some of these collaterals about cells of the posterior
column.
The terminals of the descending branches and of
those ascending branches, which do not reach the brain,
end as do the collaterals within the gray matter of the
spinal cord.
The fibers of the lateral division of the dorsal root are all very fine. The
majority are unmyelinated and can be recognized only in preparations in which
the axons are stained. A good account of their appearance in Golgi prepara-
tions has been given by Barker (1899, pp. 466-468). In Weigert preparations
we must look carefully to find the few myelinated fibers contained in this divi-
sion. But in pyridin-silver preparations great numbers of fine unmyelinated
fibers, accompanied by a few which are myelinated, can be seen to turn lateral-
ward as the root filament enters the cord. These constitute the lateral division
Fig. 71. Diagram to
illustrate the arrangement
of the ascending branches
of the dorsal root fibers
within the posterior funic-
ulus of the spinal cord.
THE NERVOUS SYSTEM
of the root and enter the dorsolateral fasciculus or tract of Lissauer (Fig. 72).'
The medial division, on the other hand, consists exclusively or almost exclu-
sively of myelinated fibers. The fibers of the lateral division of the root divide
into ascending and descending branches, both of which, however, are very
short. The ascending branch, which is the longer of the two, does not extend
at most more than the length of one or two segments in the long axis of the
cord (Ranson, 1913, 1914).
The dorsolateral fasciculus, or tract of Lissauer, lies between the apex of
the posterior column and the periphery of the cord, and varies greatly in shape
and size in the different levels of the cord (Figs. 55-58). It is composed of
Medial division of dorsal root
~^X. Fasciculus cuneatus
Dorsolateral
fasciculus
Lateral division of dorsal
root
_ Dorsal spino-
cerebellar tract
Dorsal spinocerebellar tract
\< (' ^S=fe- ^^ J
__. Ventral spino-
cerebellar tract
Lateral spino-
thalamic and
spinotectal tracts
Ventral spinothalamic
tract
Fig. 72. Diagram of the spinal cord and dorsal root, showing the divisions of the dorsal root,
the collaterals of the dorsal root fibers, and some of the connections which are established by
them.
unmyelinated and fine myelinated fibers, which are derived in part from the
lateral division of the dorsal root and in part arise from cells in the neighboring
gray matter (Fig. 63).
AFFERENT PATHS IN THE SPINAL CORD
We have been at some pains to make clear the course and distribution of
the dorsal root fibers within the spinal cord because all afferent impulses which
reach the cord are carried by them. Interoceptive fibers from the viscera,
proprioceptive fibers from the muscles, tendons, and joints, as well as extero-
ceptive fibers from the skin are included in these roots; and among the latter
group are probably several subvarieties, mediating the afferent impulses out
FIBER TRACTS OP THE SPINAL CORD 99
of which the sensations of touch, heat, cold, and pain are elaborated. An
important problem which in great measure awaits solution is this: How are the
fibers of the different functional varieties distributed in the spinal cord and
along what paths are these various types of afferent impulses carried toward
the brain?
The proprioceptive fibers, which terminate at the periphery in neuromus-
cular and neuro tendinous spindles and in Pacinian corpuscles, are known to
be myelinated. They must, therefore, pass through the well myelinated medial
division of the dorsal root into the posterior funiculus. As shown by Brown-
Sequard in 1847 by a study of patients with unilateral lesions of the spinal
cord, sensations from the muscles, joints, and tendons reach the brain without
undergoing a crossing in the spinal cord. This and other evidence points un-
mistakably to the long ascending branches of the dorsal root fibers, which are
continued uncrossed in the posterior funiculus to the medulla oblongata, as the
conductors of this type of sensation. When these fibers are destroyed by a
tumor or other lesion confined to the posterior funiculus, muscular sensibility
and the recognition of posture are abolished, while touch, pain, and tempera-
ture sensations remain intact (Dejerine, 1914).
No better exposition of the proprioceptive functions could be furnished than
by describing the sensory deficiencies found in cases of tabes dorsalis or loco-
motor ataxia, a disease in which there is degeneration of the posterior funiculi.
Lying in bed, with eyes closed, a tabetic may not be able to say in what posi-
tion his foot has been placed by an attendant because afferent impulses from
the muscles, joints, and tendons fail to reach the cerebral cortex and arouse
sensations of posture. Not only are the sensations of this variety lacking, but
the unconscious reflex motor adjustments initiated by proprioceptive afferent
impulses are also impaired. Standing with feet together and eyes closed, the
patient loses his balance and sways from side to side. In walking his gait is
uncertain and the movements of his limbs poorly coordinated. All of this
motor incoordination is explained by a loss of the controlling afferent impulses
from the muscles, joints, and tendons.
The long ascending fibers of the posterior funiculus, which reach the brain
and end in the nucleus gracilis and cuneatus, are for the most part proprio-
ceptive in function (Fig. 235). The connections which they make there can
best be considered in another chapter. Collaterals and many terminal branches
end in the gray matter of the cord, entering into synaptic relations with the neu-
rons of the spinocerebettar paths and with neurons belonging to spinal reflex arcs.
100 THE NERVOUS SYSTEM
Proprioceptive Paths to the Cerebellum. According to the researches of
Marburg (1904) and Bing (1906) the spinocerebellar tracts are concerned with
the transmission to the cerebellum of afferent impulses from the muscles, joints,
and tendons, which remain, however, at a subconscious level (Dejerine, 1914).
We may, therefore, appropriately consider these paths at this time.
The dorsal spinocerebellar tract (fasciculus spinocerebellaris dorsalis, direct
cerebellar tract of Flechsig, fasciculus cerebellospinalis) is a well-defined bundle
at the surface of the lateral funiculus just ventral to the posterior lateral sul-
cus (Figs. 72, 78). In cross-section it has the form of a flattened band, situated
between the periphery of the cord and the lateral corticospinal tract. It begins
in the upper lumbar segments and is prominent in the thoracic and cervical
portions of the cord. It consists of uniformly large fibers, which take origin
from the cells of the nucleus dorsalis of the same side. This nucleus forms a
prominent feature of the sections through the thoracic portion of the cord, but
is not found above the seventh cervical nor below the second lumbar seg-
ments. A conspicuous bundle of myelinated collaterals from fibers of the
fasciculus cuneatus run to this nucleus (Fig. 56) where their arborizations form
baskets about the individual cells of the nucleus. The fibers arising from the
cells of the nucleus dorsalis run laterally to the periphery of the lateral funiculus
of the same side, where they turn rostrally and form the dorsal spinocerebellar tract.
We will follow this tract into the brain in a latter chapter. Here we need only
say that it reaches the cerebellum by way of the restiform body (Fig. 235).
The ventral spinocerebellar tract constitutes the more superficial portion of
a large ascending bundle of fibers, known as the fasciculus anterolateralis super-
ficialis or Gower's tract, which also includes the spinotectal and lateral spino-
thalmic tracts (Fig. 72). It is situated at the periphery of the lateral funiculus
ventral to the tract we have just considered. It is said to consist of fibers which
arise from the cells of the posterior gray column and intermediate gray matter of the
same and the opposite side (Page May, 1906; Dejerine, 1914). In a subsequent
chapter we will trace these fibers by the way of the medulla, pons, and an-
terior medullary velum to the cerebellum (Fig. 235).
From what has been presented above it will be apparent that collaterals
and terminal branches of dorsal root fibers, doubtless of the proprioceptive
group, enter into synaptic relations with certain intraspinal neurons, the axons
of which run to the cerebellum by way of the ventral and dorsal spinocerebellar
tracts. The entire path from periphery to cerebellum therefore consists of two
neurons with a synaptic interruption in the gray matter.
FIBER TRACTS OF THE SPINAL CORD IOI
Interoceptive fibers are present in the thoracic and upper lumbar dorsal
roots, but are either absent or very few in number in the others. We know
practically nothing about their intraspinal course in mammals. They will be
considered in the chapter on the Sympathetic Nervous System.
Exteroceptive fibers carry cutaneous afferent impulses, and probably are
subdivided into several varieties. Most authors agree that there are separate
fibers for the impulses aroused by tactile and thermal stimuli; and Sherrington
(1906) has presented evidence for the existence of a separate group of fibers,
whose end organs are responsive only to agents capable of inflicting injury,
that is, to noxious or painful stimuli.
Conduction of Tactile Impulses in the Spinal Cord. The phenomena of sen-
sory dissociation, characteristic of syringomyelia, show that the intraspinal
path for the sensations of touch is rather widely separated from that for pain
and temperature sensation (Fig. 73). In that disease a cavity is developed
within the gray matter of the spinal cord; and sensations of pain and tem-
perature may be abolished over a given cutaneous area which is still sensitive
to touch. The separation of these two lines of conduction occurs at the place
where the dorsal root fibers enter the cord. The fibers, mediating pain and
temperature sensations, end almost at once in the gray matter, while those for
touch ascend for some distance in the posterior funiculus of the same side (Head
and Thompson, 1906; Dejerine, 1914). As these fibers ascend in the posterior
funiculus they give off collaterals to the gray matter of the successive levels of
the spinal cord through which they pass. The tactile impulses from a given
root, therefore, do not enter the gray matter all at once, but filter forward through
the collaterals and terminals of these dorsal root fibers to reach the posterior
gray column in a considerable number of segments above that at which the
root enters the cord. Within the posterior gray column at these successive
levels the terminals and collaterals of the tactile fibers establish synaptic con-
nections with neurons of the second order. The axons of these neurons form the
ventral spinothalamic tract of the opposite side (Fig. 73).
The ventral spinothalamic tract is an ascending bundle of fibers found in the
anterior funiculus. It consists of fibers which take origin from cells in the pos-
terior column of the opposite side, cross the median plane in the anterior white
commissure, and ascend in the ventral funiculus to end within the thalamus (Fig.
73). It is possible that many of the fibers do not reach the thalamus directly,
but terminate in the gray matter of the cord and medulla oblongata in rela-
tion to other neurons, whose axons continue the course to the thalamus. If
IO2
NERVOUS SYSTEM
this be so the path consists in part of relays of shorter neurons (D6jerine,
1914).
The uncrossed path in the posterior funiculus for tactile impulses entering
the cord through any given dorsal root overlaps by many segments the crossed
path in the ventral funiculus (Fig. 230). Some of the uncrossed fibers even
reach the nuclei of the funiculus gracilis and funiculus cuneatus in the medulla
oblongata. This extensive overlapping of the uncrossed by the crossed paths
accounts for the fact that lateral hemisection of the human spinal cord rarely
causes marked disturbance of tactile sensibility below the lesion (Petren, 1902;
Head and Thompson, 1906).
I
Ascending branch of dorsal root fiber -
Myelinated fiber of dorsal root^
Spinal ganglion
Unmyelinated fiber of dorsal root'
~ Lateral spinothalamic tract
(pain and temperature)
Ventral spinothalamic tract
(touch)
Fig. 73. Exteroceptive pathways in the spinal cord.
Since it seems clear that the dorsal root fibers subserving tactile sensibility ascend for
some distance in the posterior funiculus, they must be included among the myelinated fibers
of the medial division of the dorsal root, because only myelinated fibers ascend in that
funiculus. This conclusion is in keeping with the facts already mentioned concerning the
termination of myelinated fibers in the supposedly tactile end organs, such as Meissner's
corpuscles and Pacinian corpuscles. It is also in. keeping with facts to be mentioned in
a following paragraph concerning the structure of the median nerve.
The Lateral Spinothalamic Tract. It seems to be well established that the
dorsal root fibers, which serve as pain conductors, terminate in the gray matter
almost at once after entering the cord, and come into synaptic relations with
neurons of the second order, whose axons run in the lateral spinothalamic tract.
From cells in the posterior column fibers arise, which in man cross to the opposite
side of the cord in the anterior white commissure and ascend in the lateral spino-
thalamic tract to end in the thalamus (Figs. 73, 231). This is a tract of ascending
FIBER TRACTS OF THE SPINAL CORD 103
fibers situated in the lateral funiculus under cover of the ventral spinocerebellar
tract. Together with the spinotectal and ventral spinocerebellar tracts it forms
the fasciculus anterolateralis superficialis (of Gowers). It mediates pain and
temperature sensations.
Conduction of Painful Afferent Impulses in the Spinal Cord. Not all of the fibers of
the lateral spinothalamic tract reach the thalamus. According to May (1906), "Some of
these fibers certainly pass directly to the thalamus, while others terminate in the inter-
mediate gray matter, and thus, by means of a series of short chains, afford secondary paths
to the same end station, which may supplement the direct path, or be made available after
interruption of the direct path." It has been shown in many cases in man and animals that,
after a complete hemisection of the spinal cord, the loss of sensibility to pain on the op-
posite side of the body below the lesion was only temporary. In time there may occur a
more or less perfect restoration of pain conduction, showing that the homolateral side of
the cord is able to supplement or replace the heterolateral path. According to the researches
of Karplus and Kreidl (1914) and Ranson and Billingsley (1916) these short chains, which are
of secondary importance in man, are much better developed in the cat. In this animal
pain conduction through the spinal cord is bilateral and is effected to a large extent through
a series of short relays.
According to Head and Thompson (1906) the path for pain in the spinal cord is the same
whether the impulses arise in the skin or in the deeper parts, such as the muscles and joints.
But Dejerine (1914) is of the opinion that painful impulses from the muscles may be trans-
mitted in the posterior funiculus and remain uncrossed as far as the medulla oblongata.
Until recently we possessed no information as to which dorsal root fibers served as pain
conductors. But in the last few years evidence has been presented which points toward the
unmyelinated fibers of the spinal nerves and dorsal roots as the pain fibers (Ranson, 1915).
Space does not permit a detailed presentation of the evidence here. It should be noted,
however, that the unmyelinated fibers of the lateral division of the dorsal root terminate in
the gray matter almost immediately after their entrance into the spinal cord, and in this
respect correspond to the known course of the fibers carrying painful impulses. The un-
myelinated fibers are chiefly distributed in the cutaneous nerves, although a few run in the
muscular branches. This coincides with the much greater sensitiveness to pain of the
skin than of the deeper tissues. Furthermore, the median nerve at the wrist, a large nerve
supplying a relatively small area of skin richly endowed with the sense of touch, contains
relatively few unmyelinated fibers. On the other hand, nerves like the lateral cutaneous
of the thigh and the medial cutaneous of the forearm, which supply relatively large cutaneous
areas of low tactile sensibility, but not inferior to the fingers in sensitiveness to pain, are com-
posed in large part of unmyelinated fibers. This difference between the composition of the
median nerve and the medial cutaneous nerve of the forearm is just what should be expected
if the touch fibers are myelinated and the pain fibers unmyelinated. Head and his co-workers
(1905, 1906, 1908) have regarded the group of sensations (protopathic), to which according
to their classification cutaneous pain belongs, as primitive in character and the first to appear
in the phylogenetic series. It is well known that nerve-fibers in their earliest phylogenesis
are unmyelinated. If our conception is correct, a great many of the afferent fibers of mam-
mals remain in this primitive undifferentiated state and mediate a relatively primitive
form of sensation. In this connection it is interesting to note that Dejerine (1914) believes
that pain is conducted by the "sympathetic" fibers contained in the cutaneous and muscular
nerves. He does not state the evidence on which this belief is based; but if by "sympathetic"
he means to designate the unmyelinated fibers his view agrees perfectly with that presented
in the preceding paragraphs.
104
THE NERVOUS SYSTEM
The problem can be approached from the. experimental standpoint. The seventh lum-
bar dorsal root of the cat is especially adapted for such a test. This root as it approaches
the cord breaks up into a number of filaments which spread out in a longitudinal direction
and enter the cord along the posterolateral sulcus. Within each root filament, as it ap-
proaches this sulcus, the unmyelinated separate out from among the myelinated fibers and
take up a position around the circumference of the filament and along septa that divide it
into smaller bundles. As the root enters the cord, these unmyelinated fibers turn laterally
into the dorsolateral fasciculus, constituting together with a few fine myelinated fibers the
lateral division of the root (Fig. 74). Almost all of the myelinated fibers run through the
medial division of the root into the cuneate fasciculus. A slight cut in the direction of the
Exterior fin\lculus.
Inmuel 'mated [tiers.
Lissauers tract
.Dorsal Toot
qela/tinosa.
Lateral
funiculus
Fig. 74. From a section of the seventh lumbar segment of the spinal cord of the cat, showing the
unmyelinated fibers of the dorsal root entering the tract of Lissauer.
arrow, which as shown by subsequent microscopic examination divided the lateral without
injury to the medial division of the root, at once eliminated the pain reflexes obtainable
from this root in the anesthetized cat, such as struggling, acceleration of respiration, and
rise of blood-pressure. On the other hand, a long deep cut in the plane indicated by B,
Fig. 74, which severed the medial division of the root as it entered the cord, had little or no
effect on the pain reflexes. This series of experiments, the details of which are given else-
where (Ranson and Billingsley, 1916), furnishes strong evidence that painful afferent im-
pulses are carried by the unmyelinated fibers of the lateral division of the dorsal root.
These fibers probably terminate in the substantia gelatinosa Rolandi, and, if so, it is
not unlikely that intermediate neurons are intercalated between them and the neurons
whose axons run in the ventral spinothalamic tra.ct.
FIBER TRACTS OF THE SPINAL CORD 105
The Conduction of Sensations of Pain, of Heat, and of Cold. It is well estab-
lished on the basis of clinical observations that the paths for sensations of heat
and cold follow closely those for pain. They pass through the gray matter im-
mediately after entering the cord, cross to the opposite side, and ascend in the
lateral spinothalamic tract.
According to May (1906) "it is clear that there are distinct and separate
paths for the impulses of pain, of heat, or of cold in the spinal cord, and that
these different and specific qualities of sensation may be dissociated in an affec-
tion of the spinal cord." That is, one of these forms of sensibility may be lost,
although the other two are retained. "But as these paths are anatomically
very closely associated from origin to termination these three forms of sensa-
tion are usually affected to a like degree."
From what has been said above it will be apparent that the paths, mediating
pain and temperature sensibility, cross promptly to the opposite side of the
cord and ascend in the lateral spinothalamic tract. The path for touch crosses
more gradually, but finally comes to lie in the ventral spinothalamic tract of
the opposite side; while the sensory impulses from the muscles, joints, and
tendons, as well as some elements of tactile sensibility, are carried upward on
the same side of the cord by the long ascending branches of the dorsal root fibers,
which terminate in the nuclei of the funiculus gracilis and the funiculus cuneatus.
The connections established within the brain by the fibers of these various paths
cannot profitably be discussed at this point, but will be considered in Chapter XIX.
Other afferent paths besides those already mentioned exist in the spinal
cord. These include the spino-olivary and spinotectal tracts (Fig. 78). The
former consists of fibers which arise from cells in the posterior gray column,
cross to the opposite side of the cord, and ascend in the ventral funiculus, to
end in the inferior olivary nucleus of the medulla oblongata. The spinotectal
tract consists of fibers which arise from cells in the posterior gray column and
which, after crossing, ascend in the lateral funiculus in company with those of
the lateral spinothalamic path to end in the roof (tectum) of the mesencephalon,
i. e., in the corpora quadrigemina.
ASCENDING AND DESCENDING DEGENERATION OF THE SPINAL CORD
When as a result of an injury a nerve-fiber is divided, that part which is
severed from its cell of origin degenerates, while the part still connected with
that cell usually remains intact. This is known as Wallerian degeneration, and,
as will be readily understood, gives valuable information concerning the course
io6
THE NERVOUS SYSTEM
of the fiber tracts. In case of a complete transection of the spinal cord all the
ascending fibers whose cells are located below the cut will degenerate in the
segments above; while those descending fibers whose cells of origin are located
above will degenerate below the lesion (Fig. 75). Injury to the dorsal roots
proximal to the spinal ganglia causes a degeneration of the dorsal root fibers
Dorsal spinocerebellar tract
f .fCorticospinal tract
i
"Ascending branches of dorsal root fibers
Fasciculus proprius
Descending branch of dorsal root fiber
Fig. 75. Diagram of the spinal cord to illustrate the principle of Wallerian degeneration.
The broken lines represent the degeneration resulting from 1, section of the ventral root; 2,
section of the spinal nerve distal to the spinal ganglion; 3, section of the dorsal root proximal to
the spinal ganglion, and 4, a lesion in the lateral funiculus.
throughout their length in the spinal cord. Brain injuries may, according to
their location, result in the degeneration of one or more of the tracts which
descend into the spinal cord from above.
By the study of a great many cases of injury to the central nervous system
in man and of experimentally produced lesions in animals a very considerable
FIBER TRACTS OF THE SPINAL CORD
107
amount of information has been obtained concerning the fiber tracts of the
spinal cord (Collier and Buzzard, 1901, 1903; Stewart, 1901; Thiele and Horsley,
1901 ; Batten and Holmes, 1913). This is summarized in the accompanying table
and in Fig. 78.
TABLE SHOWING THE LOCATION OF THE CHIEF FIBER TRACTS OF THE SPINAL CORD AND THE
DIRECTION IN WHICH THEY DEGENERATE
Ascending degeneration.
Descending degeneration.
Anterior funiculus
Ventral spinothalamic tract
Ventral corticospinal tract,
Vestibulospinal tract,
Tectospinal tract
Lateral funiculus
Dorsal spinocerebellar tract,
Ventral spinocerebellar tract,
Lateral spinothalamic tract,
Spinotectal tract
Lateral corticospinal tract,
Rubrospinal tract,
Bulbospinal tract,
Tectospinal tract
Posterior funiculus
Ascending branches of the
dorsal root fibers
Fasciculus interfascicularis,
Septomarginal tract
The fasciculi proprii or ground bundles are composed of short ascending
and descending fibers, which arise and terminate within the gray matter of the
spinal cord and link together the various segments of the cord. These fascicles,
one of which is present in each of the three funiculi, immediately surround
the gray columns. After a transection of the spinal cord the fasciculi proprii
undergo an incomplete degeneration for some distance both above and below
the lesion (Figs. 75, 76). In cross-section the ground bundle of the posterior
funiculus has the form of a narrow band upon the surface of the posterior column
and posterior commissure, and was once called the cornu-commissural bundle
(Fig. 78). In addition to this fascicle there are in the posterior funiculus two
other tracts which in part belong to the same system the septomarginal tract
and the fasciculus interfascicularis, or comma tract of Schultze. These are
both composed of descending fibers, in part of intraspinal origin and in part
representing the descending branches of the dorsal root fibers. The septomar-
ginal tract is situated along the dorsal periphery of the posterior funiculus in
the thoracic region; it takes up a position along the septum in the lumbar segments
(oval area of Flechsig) ; and in the sacral region it forms a triangular field at the
dorsomedial angle of the posterior funiculus (triangle of Gombault and Philippe)
(Fig. 76). The fasciculus interfascicularis is best developed in the thoracic
segments, where it occupies a position near the center of the posterior funiculus.
io8
THE NERVOUS SYSTEM
In the anterior funiculus, in addition to the fasciculus proprius which imme-
diately surrounds the gray matter, there is a thin layer of similar fibers spread
out along the border of the anterior fissure and known as the sulcomarginal
fasciculus. This tract also contains the fibers which descend into the cord from
the medial longitudinal bundle of the medulla oblongata.
As a general rule the short fibers of the fasciculus proprius lie nearer the
gray substance than the fibers of greater length; and the long tracts, which
Fasciculus gracilis ..,
Spinocerebellar, spinotectal, and lateral^
spinothalnmic tracts
Fasciculus inter fascicular is
Septomarginal fasciculus-
Lateral corticospinal tract*
Septomarginal fasciculus, oval area of Flechsig ,
Lateral corticospinal tract*
Cervical enlargement
ascending degeneration
Upper thoracic
ascending degeneration
Middle thoracic
site of compression
Lower thoracic
descending degeneration
Upper lumbar
descending degeneration
Lower lumbar
descending degeneration
Fig. 76. Ascending and descending degeneration resulting from a compression of the thoracic
spinal cord in man. Marchi method. (Hoche.)
connect the spinal cord with the brain, occupy the most peripheral position.
But the fact must not be overlooked that many fibers of the fasciculus proprius
are intermingled with those of the long tracts.
LONG DESCENDING TRACTS OF THE SPINAL CORD
Fibers which arise from cells in various parts of the brain descend into the
spinal cord, where they form several well-defined tracts. The most important
FIBER TRACTS OF THE SPINAL CORD 109
and most conspicuous of these are the cerebrospinal fasciculi, which are more
properly called the corticospinal tracts. There are two in each lateral half of
the cord, the lateral and the ventral corticospinal tracts. Their constituent
fibers take origin from the large pyramidal cells of the precentral gyrus or motor
region of the cerebral cortex and pass through the subjacent levels of the brain
to reach the spinal cord (Fig. 77). Just before they enter the spinal cord they
undergo an incomplete decussation in the medulla oblongata, giving rise to a
ventral and a lateral corticospinal tract.
The Lateral Corticospinal Tract (Crossed Pyramidal Tract, Fasciculus
Cerebrospinalis Lateralis). The majority of the pyramidal fibers, after cross-
ing the median plane in the decussation of the pyramids, enter the lateral fu-
Cerebral hemisphere
Spinal
cord
Fig. 77. Diagram of the corticospinal tracts.
niculus of the spinal cord as the lateral corticospinal tract, which occupies a posi-
tion between the dorsal spinocerebellar tract and the lateral fasciculus proprius
(Fig. 78). In the lumbar and sacral regions, below the origin ot the dorsal
spinocerebellar tract, the lateral corticospinal tract is more superficial. It can
be traced as a distinct strand as far as the fourth sacral segment; and as it
descends in the spinal cord it gradually decreases in size. Throughout its
course in the spinal cord it gives off collateral and terminal fibers which end in
the gray matter.
The ventral corticospinal tract (fasciculus cerebrospinalis anterior or direct
pyramidal tract) is formed by the smaller part of the corticospinal fibers, which
do not cross in the medulla, but pass directly into the ventral funiculus of the
no
THE NERVOUS SYSTEM
same side of the cord. They form a tract of small size, which lies near the
anterior median fissure and which can be traced as a distinct strand as far as the
middle of the thoracic region of the spinal cord. Just before terminating these
fibers cross in the anterior white commissure. They end like those of the lateral
corticospinal tract, either directly or perhaps through an intercalated neuron,
in relation to the motor cells in the anterior column. The crossing of these
fibers is only delayed, and it will be apparent that all of the corticospinal fibers
arising in the right cerebral hemisphere terminate in the anterior column of the
left side of the cord, and conversely, those from the left hemisphere end on the
right side. It is along these fibers that impulses from the motor portion of the
cerebral cortex reach the cord and bring the spinal motor apparatus under
voluntary control.
Fasciculus septomarginalis
Fasciculus inlerfascicularis
Fasciculus proprius
Sensory fibers of the
second order ~
Lateral corticospinal _
tract
Rubrospinal tract I-
Tectospinal tract - -
Fasciculus proprius NT
Bulbospinal tract
Vestibules pinal tract--,
Fasciculus gracilis
.,-- Fasciculus cuneatus
- - - Dorsolateral fasciculus
^_ Dorsal spinocerebellar
tract
~~ Fasciculus proprius
Ventral spinocere-
bellar tract
jf-.. Lateral s pinotltalamic
tract
Spinotectal tract
- Ventral root
~~ Ventral spinolhalamic tract
* T . " Sulcomareinal fasciculus
Ventral cortjcosptnal tract
Fig. 78. Diagram showing the location of the principal fiber tracts in the spinal cord of man.
Ascending tracts on the right side, descending tracts on the left.
It is stated by some authors, although on the basis of rather unsatisfactory evidence,
that the fibers of the lateral corticospinal tract ramify in the formatio reticularis (Mona-
kow, 1895) and the nucleus dorsalis (Schafer, 1899). The corticospinal path is from the
standpoint of phylogenesis a relatively new system and varies a great deal in different
mammals. It is found in the ventral funiculus in the mole, while in the rat it occupies the
posterior funiculus. In the mole it is almost completely unmyelinated, in the rat largely so.
It contains many unmyelinated fibers in the cat, fewer in the monkey (Linowiecki, 1914).
In man it does not become fully myelinated before the second year. An uncrossed ventral
corticospinal tract seems to be present only in man and the anthropoid apes, and this tract
varies greatly in size in different individuals.
The rubrospinal tract (tract of Monakow) is situated near the center of the
lateral funiculus just ventral to the lateral corticospinal tract (Fig. 78). Its
fibers come from the red nucleus of the mesencephalon, cross the median plane,
FIBER TRACTS OF THE SPINAL CORD
III
and descend into the spinal cord, within which some of them can be traced to
the sacral region. Their collateral and terminal branches end within the an-
terior column in relation to the primary motor neurons.
Other Descending Tracts. The bulbospinal tract (olivospinal tract, tract of
Helweg) is a small bundle of fibers found in the cervical region near the surface
of the lateral funiculus opposite the anterior column. The fibers arise from
cells in the medulla oblongata, possibly in the inferior olivary nucleus, and end
somewhere in the gray matter of the spinal cord. The exact origin and ter-
Fasciculus cuneatus
\
Fasciculus gracilis
Lateral corticospinal tract
Fasciculi proprii
Ventral corticospinal tract
- Dorsal spinocerebellar tract
Oval area of Flechsig
D. Ill
L. IV
Fig. 80.
Figs. 79 and 80. Diagrams of the sixth cervical, third thoracic, and fourth lumbar segments
of the spinal cord, showing the location of the different tracts as outlined by Flechsig on the basis
of differences in time of myelination. (van Gehuchten.)
mination of the tract is unknown. The tectospinal tract, located in the ventral
funiculus, is composed of fibers which take origin in the roof (tectum) of the
mesencephalon, cross the median plane and descend into the anterior funiculus
of the spinal cord, and end in the gray matter of the anterior column. The tract
is concerned chiefly with optic reflexes. The vestibulospinal tract, also located
in the anterior funiculus, arises from the lateral nucleus of the vestibular nerve
112 THE NERVOUS SYSTEM
in the medulla oblongata and conveys impulses concerned in the maintenance
of equilibrium. Some of its fibers can be traced as far as the lower lumbar
segments. They end in the gray matter of the anterior column.
Hemisection of the spinal cord in man produces a characteristic symptom
complex known as the Brown-Sequard's syndrome which the student is now in
position to understand. Below the level of the lesion and on the same side
there is found a paralysis of the muscles with a loss of sensation from the mus-
cles, joints, and tendons; while on the opposite side of the body, beginning two
or three segments below the level of the lesion, there is loss of sensations of
pain and temperature. Tactile sensibility is everywhere retained (Dejerine,
1914).
Order of Myelination. The fiber tracts of the spinal cord do not all become
myelinated at the same time. By a study of the fetal spinal cord at various
developmental stages Flechsig was able to identify and trace many of these
tracts because of the difference in the tune of myelination. His results agree
in general with those derived frorh a study of spinal cords showing ascending
and descending degeneration (Figs. 79, 80). Myelination begins during the fifth
month of intra-uterine life. The order in which the fibers of the spinal cord
acquire their myelin sheaths is as follows: (1) afferent and efferent root fibers,
(2) those of the fasciculi proprii, (3) the fasciculus cuneatus, (4) the fasciculus
gracilis, (5) the dorsal spinocerebellar tract, (6) the ventral spinocerebellar fas-
ciculus, (7) the corticospinal tracts.
CHAPTER VIII
THE GENERAL TOPOGRAPHY OF THE BRAIN. THE EXTERNAL
FORM OF THE MEDULLA OBLONGATA, PONS, AND MESEN-
CEPHALON
The General Topography of the Brain. The brain rests upon the floor of
the cranial cavity, which presents three well-marked fossae. In the posterior
cranial fossa are lodged the medulla oblongata, pons, and cerebellum, which
together constitute the rhombencephalon (Fig. 81). This fossa is roofed over
by a partition of dura mater, called the tentorium cerebelli, that separates the
cerebellum from the cerebral hemispheres. Through the notch in the ventral
Calvaria
Prosen-( Telencephalon
cephalon\Diencephalon
Frontal lobe of cerebral
hemisphere in anterior
cranial fossa
Temporal lobe of cerebral
hemisphere in middle
cranialfossa
Parietal lobe of cerebral
hemisphere
Mesencephalon
Occipital lobe of cerebral
hemisphere
Tentorium cerebelli
Posterior cranialfossa
Cerebellum
Pons
Medulla oblongata
Spinal cord
Fig. 81. Median sagittal section of the head showing the relation of the brain to the cra-
nium. The sphenoid bone is shown in transparency, and through it the temporal lobe may be
seen.
border of the tentorium projects the mesencephalon, connecting the rhomben-
cephalon below with the prosencephalon above that partition. The cerebral
hemispheres form the largest part of the prosencephalon, occupy the anterior
and middle cranial fossae, and extend to the occiput on the upper surface of the
tentorium.
The dorsal aspect of the human brain presents an ovoid figure, the large
cerebral hemispheres, covering the other parts from view. In the sheep's brain the
8 113
THE NERVOUS SYSTEM
hemispheres are smaller and fail to hide the cerebellum and medulla oblongata
(Fig. 82). The cerebral hemispheres, which are separated by a deep cleft called
the longitudinal fissure of the cerebrum, together present a broad convex surface
which lies in close relation to the internal aspect of the calvaria. From the
latter it is separated only by the investing membranes or meninges of the brain.
The thin convoluted layer of gray matter upon the surface of the hemispheres is
known as the cerebral cortex.
The ventral aspect or base of the brain presents an irregular surface adapted
to the uneven floor of the cranial cavity (Figs. 83, 86). The medulla oblongata,
Face and tongue
Head and eyes
Fore limb
Hind limb
Gyrus sylviactis (arcuatus)
Cyrus lateralis
Gvri mediates
Gyrus internus I / /-
Vermis cerebelli
Hemisptuerium cerebelli
Medulla oblongata
Medulla spinalis
Gyrus frontalis medialis
Gyrus frontalis superior
Sulcus coronal is
Sulcus splenialis
Fissura ansata (cruciata)
Fissura lateralis (Sylvii)
Fissura suprasylvia
Fissura longitudinalis
Sulcus lateralis
Sulcus intermedius
Sulcus medialis
Flocculus
Neruus accessorius
Nervus spinalis I
Fig. 82. Dorsal view of the sheep's brain. The motor cortex is shaded on the left side. (Herrick
and Crosby.)
which is continuous through the foramen magnum with the spinal cord, lies on
the ventral aspect of the cerebellum in the vallecula between the two cere-
bellar hemispheres. Rostral to the medulla oblongata and separated from it
only by a transverse groove is a broad elevated band of fibers, which plunges
into the cerebellum on either side and is known as the pons. The cerebellum
can be seen occupying a position dorsal to the pons and medulla oblongata, and
can easily be recognized by its grayish color and many parallel fissures. A
pair of large rope-like strands are seen to emerge from the rostral border of
the pons and to diverge from each other as they run toward the under surface
THE GENERAL TOPOGRAPHY OF THE BRAIN
of the cerebral hemispheres. These are the cerebral peduncles and they form
the ventral part of the mesencephalon. At its rostral extremity each peduncle
is partially encircled by a flattened band, known as the optic tract, which is con-
tinuous through the optic chiasma with the optic nerves. A lozenge-shaped
depression, known as the inter peduncular fossa, is outlined by the diverging
cerebral peduncles and by the optic chiasma and tracts. Within the area thus
outlined and beginning at its caudal angle may be distinguished the following
parts: the inter peduncular nucleus, which is very large in the sheep and occu-
Longitudinal fissure of cerebrum^
Optic nerve^
Optic chiasma
Rhinal fissure
Insula-
Lateral fissure
Optic tract .
Infundibulum -~
Mammittary body -
Cerebral peduncle
Inter peduncular fossa and
nucleus
Trigeminal nerve
Abducens nerve---
Acoustk( Vestibular n ~
nene (Cochlearn.
Glossopharyngeal nerve ~-'
Vagus nerve
Hypoglossal nerve ''
Anterior median fissure''
' Olfactory bulb
' Medial olfactory gyrus
Anterior perforated substance
- Lateral olfactory stria
Lateral olfactory gyrus
-Diagonal band
..- Amygdaloid nucleus
Pyriform area
; Hippocampal gyrus
L-- Trochlear neroe
m,,.--Pons
Jn. .-'A bducens nerve
~*?^,_-- Facial nerve
Trapezoid body
Cerebellum
'---Olive
^Chorioid plexus
" Accessory nerve
^Tractus later alis minor
Fig. 83. Ventral view of the sheep's brain.
pies an area designated in man as the substantia perforata posterior; the corpus
mammillare, which in man is divided by a longitudinal groove into two mam-
millary bodies; and also the tuber cinereum, infundibulum, and hypophysis.
Rostral to the optic tract there is on either side a triangular field of gray matter,
studded with minute pit-like depressions and known as the anterior perforated
substance.
The Rhinencephalon. The olfactory bulb is situated near the rostral end
of the hemisphere, to the ventral surface of which it is attached by the olfactory
n6
THE NERVOUS SYSTEM
peduncle (and in man by the long olfactory tract). In the sheep's brain there
diverge from the olfactory peduncle two well-defined gray bands, the medial
and lateral olfactory gyri, which are less evident in man; and furthermore, the
lateral olfactory gyrus is obviously continuous with the hippocampal gyrus,
forming the pyriform area (Fig. 83). All of these structures are closely asso-
ciated in function and belong to the rhmencephalon, or olfactory part of the
brain, which, because of the greater importance of the sense of smell in the
sheep, is better developed in that animal than in man. A prominent longi-
tudinal fissure separates this part of the brain from the rest of the hemisphere.
Inter-ventricular foramen Body of corpus callosum
Anterior commissure
Septum pellucidum^
Rostral lamina
Rostrum of corpus callosum. \
Genu of corpus callosum { \ '
Body of fornix
\ Hippocampal com. Roofs of third ventricle or tela choriotdea
Stria med. /Haben. com.
Splenium
fineal
body
Suprapineal recess
', Superior colliculus
' -Primary fissure
White center of vermis
Olfactory bulb
Medial olfactory gyrus ,
Anterior perf. substance';
Lamina terminalis
Diagonal band
>'/ ! / / ! Infundib. \
' / / ,' Third vent.
! ' Massa intermedia
i Optic chiasma
Preoptic recess
\ ' \ \ 'Pons
\ \ 'Aqueduct
* \Lamina quad.
\ 'Posterior com.
\ * Hypophysis
Mammillary body
Central canal
\ Medulla
\ Medial aperture of
\ \ fourth ventricle
\ \Tela chorioidea
\ * Fourth ventricle
''Anterior medullary
velum
Fig. 84. Medial sagittal section of the sheep's brain.
This is known as the rhinal fissure; and all that portion of the cerebral cortex
which lies dorsal to it is the new or non-olfactory cortex, the neopattium. In
contrast to the older olfactory cortex or archipallium, which includes the pyri-
form area, the neopallium is of recent phyletic development. It first forms a
prominent part of the brain in mammals and is by far the most highly developed
in man.
Interrelation of the Various Parts of the Brain. An examination of a medial
sagittal section of the brain will make clear the relation which the various parts
bear to each other (Fig. 84). The medulla oblongata, pans, and cerebellum are
seen surrounding the fourth ventricle, and are intimately connected with one
THE GENERAL TOPOGRAPHY OF THE BRAIN 117
another. The medulla oblongata is directly continuous with the pons, and on
either side a large bundle of fibers from the dorsal aspect of the former runs into
the cerebellum. These two strands, which are known as the restiform bodies
or inferior cerebellar peduncles, constitute the chief avenues of communication
between the spinal cord and medulla oblongata on the one hand and the cere-
bellum on the other. The ventral prominence of the pons is produced in large
part by transverse bundles of fibers, which when traced lateralward are seen to
form a large strand, the brachium pontis or middle cerebellar peduncle, that
enters the corresponding cerebellar hemisphere (Figs. 83, 86). The brachium
conjunctivum or superior cerebellar peduncle can be traced rostrally from the
cerebellum to the mesencephalon. The three peduncles are paired structures,
symmetrically placed on the two sides of the brain (Figs. 87, 88).
The Cerebrum. The mesencephalon surrounds the cerebral aqueduct and
consists of the ventrally placed cerebral peduncles, and a dorsal plate with four
rounded elevations, the lamina and corpora quadrigemina (superior and inferior
colliculi). The cerebral hemispheres form the most prominent part of the
cerebrum and are separated from each other by the longitudinal fissure (Fig.
82), at the bottom of which is a broad commissural band, the corpus callosum,
which joins the two hemispheres together (Fig. 85). Under cover of the cere-
bral hemispheres and concealed by them, except on the ventral aspect of the
brain, is the diencephalon. This includes most of the parts which help to form
the walls of the third ventricle. These are from above downward, the epithal-
amus, including the habenular trigone and pineal body near the roof of the
ventricle; the thalamus, which forms most of the lateral wall of the ventricle,
and is united with its fellow across the cavity by a short bar of gray substance,
the massa intermedia; and the hypothalamus, including the mammillary bodies,
infundibulum, and part of the hypophysis (Figs. 84, 85).
The Brain Ventricles. The central canal of the spinal cord is prolonged
through the caudal portion of the medulla oblongata and finally opens out into
the broad rhomboidal fourth ventricle of the rhombencephalon. At its pointed
rostral extremity this ventricle is continuous with the cerebral aqueduct, the
elongated slender cavity of the mesencephalon. This, in turn, opens into the
third ventricle, which is a narrow vertical cleft between the two laterally sym-
metric halves of the diencephalon. It is bridged by the massa intermedia.
Near the dorsal part of the rostral border of the ventricle is a small opening in
each lateral wall, the inter-ventricular foramen or foramen of Monro. This
leads into the lateral ventricle, the cavity of the cerebral hemisphere.
THE NERVOUS SYSTEM
THE ANATOMY OF THE MEDULLA OBLONGATA
At its rostral end the spinal cord increases in size and goes over without
sharp line of demarcation into the medulla oblongata, or myelencephalon, which,
as we learned in Chapter II, is derived from the posterior part of the third brain
vesicle. The medulla oblongata may be said to begin just rostral to the high-
est rootlet of the first cervical nerve at about the level of the foramen magnum ;
Marginal part of sukus cinguli
Sulcus of corpus callosum \
Splenium of corpus callosum \ ;'
Precuneux
Sub parietal sulcus \
Parieto-occipital fissure\
Lamina quadrigemina
Cuneus
Superior vermis^
Calcarinefissure f\
Occipital pole f
Lingual gyrus
Transverse fissure
Cerebellar hem.
Medullary substance
of vermis
Inferior vermis''
Calamus scriptorius''
Central canal \
Spinal cord , ,
Tela chorioidea of fourth ventricle f
Fourth ventricle ;
Medulla oblongata
Anterior medullary velum
Cerebral aqueduct !
Pans ! ;
Posterior perforated substance ,
Oculomotor nerve
Central sulcus in paracentral lobule
Pineal body
I Pineal recess
! ', Posterior commissure
I I 1 Tela chorioidea of third ventricle
Massa intermedia
. Gyrus cinguli
Thalamus
Body of corpus callosum
Body offornix
Septum pellucidum
' Sulcus cinguli
Interventric. foramen
Column offornix
Anterior commis-
;> Superior frontal
gyrus
'-Frontal pole
Cenu of cor pus callosum
Nostrum of cor p. callosum
-- Parolfactory area and sulci
, X X X X X \ % \ \ s- Subcallosal gyrus
X X X X X \ \ \ Hypothalamic sulcus
X X X X \ \ X ^Lamina terminal is
X X X X \ X Optic recess
X X X X \ "Optic nerve
X X X \ Optic chiasma
X X X Infundibulum
X \ "Anterior lobe\ ,
Posterior lobe
"Mammillary body
Fig. 85. Medial sagittal section of the human brain. (Sobotta-McMurrich.)
and at the opposite extremity it is separated from the pons by a horizontal groove
(Figs. 81, 85). Its ventral surface rests upon the basilar portion of the occipital
bone; while its dorsal surface is in large part covered by the cerebellum. The
shape of the medulla oblongata is roughly that of a truncated cone, the smaller
end of which is directed caudally and is continuous with the spinal cord. In
man it measures about 3 cm., or a little more than 1 inch, in length (Fig. 86).
Like the spinal cord, the medulla oblongata presents a number of more or
ANATOMY OF THE MEDULLA OBLONGATA 119
less parallel longitudinal grooves. These are the anterior and posterior median
fissures, and a pair each of anterior lateral and posterior lateral sulci (Figs. 86,
89). By means of the fissures it is divided symmetrically into right and left
halves; while these, in turn, are marked off by the sulci into ventral, lateral, and
dorsal areas, which as seen from the surface appear to be the direct upward con-
tinuation of the anterior, lateral, and posterior funiculi of the spinal cord.
But, as we shall see in the following chapter, this continuity is not as perfect
as it appears from the surface; because the tracts of the cord undergo a rear-
rangement as they enter the medulla oblongata. The posterior median fissure
does not extend beyond the middle of the oblongata, at which point its lips
separate to form the lateral boundaries of the caudal portion of the fourth ven-
tricle. The caudal half of the medulla oblongata contains a canal, the direct
continuation of the central canal of the spinal cord, and is known as the closed
portion of the medulla oblongata (Fig. 85). This canal opens out into the fourth
ventricle in the rostral half, which helps to form the ventricular floor, and which
is often spoken of as the open part of the medulla oblongata.*
Fissures and Sulci. The posterior median fissure represents the continua-
tion of the posterior median sulcus of the spinal cord and, as noted above, ends
near the middle of the medulla oblongata. The anterior median fissure is con-
tinued from the spinal cord to the border of the pons, where it ends abruptly
in a pit known as the for amen ccecum. Near the caudal extremity of the medulla
oblongata this fissure is interrupted by interdigitating bundles of fibers which
pass obliquely across the median plane. These are the fibers of the lateral
corticospinal tract, which undergo a decussation on passing from the medulla
oblongata into the spinal cord, known as the decussation of the pyramids. The
anterior lateral sulcus also extends throughout the length of the medulla ob-
longata and represents the upward continuation of a much more indefinite groove
bearing the same name in the spinal cord. From it emerge the root filaments
of the'hypoglossal nerve. From the posterior lateral sulcus emerge the rootlets
of the glossopharyngeal, vagus, and accessory nerves (Figs. 86, 88, 89).
The ventral area of the medulla oblongata is included between the anterior
median fissure and the anterior lateral sulcus, and has the false appearance of
being a direct continuation of the anterior funiculus of the spinal cord. On
either side of the anterior median fissure there is an elongated eminence, taper-
ing toward the spinal cord, and known as the pyramid (pyramis Fig. 86). It
is formed by the fibers of the corticospinal or pyramidal tract. Just before the
fibers of this tract enter the spinal cord they undergo a more or less complete
I2O
THE NERVOUS SYSTEM
decussation, crossing the median plane in large obliquely interdigitating bundles,
which fill up and almost obliterate the anterior median fissure in the caudal
part of the medulla oblongata. This is known as the decussation of the pyra-
mids (decussatio pyramidum). In the sheep these fibers pass into the opposite
posterior funiculus of the spinal cord. In man the crossing is incomplete, a
Infundibidum
Orbital sulci of frontal
Orbital gyri of frontal lobe
Hypophysis
Temporal pole
Anterior perfor, substance
Oculomotor nerve ^
Uncus --,
Mammillary body
Cerebral peduncle -
Pans -
Trigeminal nerve -
Temporal lobe
Facial nerve
Frontal pole olfadory sukus
,. Olfactory bulb
Olfactory tract
Optic nerve
Nervus intermedius -
Acoustic nerve."
Flocculus of cerebellum^'
Cerebellum '
Chorioid plexus of ventricle IV
Glossopharyngeal nerve
Vagus nerve'
Hypoglossal nerve
Accessory nerve '
Root filaments of cervical nerve I
Decussation of pyramids
.-Optic chiasma
- Lateral olfactory stria
Tuber cinereum
Maxillary nerve
Ophthalmic nerve
Portia minor of trigem.
nerve
Mandibular nerve
Semilunar ganglion
Trochlear nerve
Inter peduncular fossa
Abducens nerve
'Olive
Pyramid
Medulla oblongata
Tonsil of cerebellum
* Occipital pole
Spinal cord
Vermis of cerebellum
Fig. 86. Ventral view of the human brain. (Sobotta-McMurrich.)
majority of the fibers descending into the lateral funiculus of the opposite side,
a minority into the anterior funiculus of the same side (Fig. 77). We are al-
ready acquainted with these bundles in the spinal cord as the ventral and lateral
corticospinal tracts (direct and crossed pyramidal tracts). In addition to the
pyramid the ventral area of the medulla also contains a bundle of fibers, the
ANATOMY OF THE MEDULLA OBLONGATA 121
medial longitudinal fasciculus, which is continuous with the anterior fasciculus
proprius of the spinal cord.
The lateral area of the medulla oblongata, included between the antero-
lateral and posterolateral sulci, appears as a direct continuation of the lateral
funiculus of the spinal cord; but, as a matter of fact, many of the fibers of that
funiculus find their way into the anterior area (as, for example, the lateral cor-
ticospinal tract) or into the posterior area (dorsal spinocerebellar tract). In
the rostral part of the lateral area, between the root filaments of the glosso-
pharyngeal and vagus nerves, on the one hand, and those of the hypoglossal,
on the other, is an oval eminence, the olive (oliva, olivary body), which is pro-
duced by a large irregular mass of gray substance, the inferior olivary nucleus,
located just beneath the surface (Figs. 87, 88). By a careful inspection of the
surface of the medulla oblongata it is possible to distinguish numerous fine
bundles of fibers, which emerge from the anterior median fissure or from the
groove between the pyramid and the olive and run dorsally upon the surface
of the medulla to enter the restiform bodies. These are the ventral external
arcuate fibers and are most conspicuous on the surface of the olive (Fig. 88) .
In the sheep there are two superficial bands of fibers not seen in the human
brain. Placed transversely near the caudal border of the pons is a belt-like
elevation, known as the trapezoid body, through which emerge the roots of the
abducens and facial nerves (Figs. 83, 87). In man the much larger pons covers
this band from view and the sixth and seventh nerves emerge from under the
caudal border of the pons. Another bundle, beginning on the ventral sur-
face of the trapezoid body near the seventh nerve, describes a graceful curve
around the ventral border of the olive and becomes lost in the lateral area of
the medulla oblongata. This has been called the fasciculus lateralis minor.
The dorsal area of the medulla oblongata is bounded ventrally by the pos-
terolateral sulcus and emergent root filaments of the glossopharyngeal, vagus,
and accessory nerves. In the closed part of the medulla oblongata it extends
to the posterior median fissure, while in the open part its dorsal boundary is
formed by the lateral margin of the floor of the fourth ventricle. The caudal
portion of this area is, in reality, as it appears, the direct continuation of the
posterior funiculus of the spinal cord. On the dorsal aspect of the medulla
oblongata the fasciculus cuneatus and fasciculus gracilis of the cord are con-
tinued as the funiculus cuneatus and funiculus gracilis, which soon enlarge into
elongated .eminences, known respectively as the cuneate tubercle and the clava
(Figs. 89, 91). These enlargements are produced by gray masses, the nucleus
122
THE NERVOUS SYSTEM
gracilis and nucleus cuneatus, within which end the fibers of the corresponding
fasciculi of the spinal cord. The clava and cuneate tubercle are displaced lat-
erally by the caudal angle of the fourth ventricle. Somewhat rostral to the mid-
dle of the medulla oblongata they gradually give place to the restiform body.
More laterally, between the cuneate funiculus and tubercle on the one hand
and the roots of the glossopharyngeal, vagus, and accessory nerves on the other,
is a third longitudinal club-shaped elevation called the tuber culum cinereum.
It is produced by a tract of descending fibers, derived from the sensory root of
the trigeminal nerve, and by an elongated mass of substantia gelatinosa which
Corona
Lentiform nucleus
Lateral geniculale body v
Medial geniculale body
Optic radiation \
Corona radiata ,
Pulvinar\
Inferior quadrigeminal brachium^*
Superior colliculus - v
Trochlear nerve O
Inferior colliculus-^
Brachium pontis'*-
Brachium conjunctivum''
Restiform body
Acoustic nerve
\ Cochtear n _
Dorsal cochlear nucleus
Glossopharyngeal nerve -
Vagus nerve and restiform body
Accessory nerve " -
Clava'"" .
Cuneate tubercle'"'
Anterior perforated substance
Optic tract
Optic nerve
Infundibulum
Mammillary body
Hypophysis
'Oculomotor nerve
" Transverse peduncular tract
Cerebral peduncle
Pans
Abducens nerve
Trigeminal nerve
Facial nerve
Trapezoid body
'* Olive
" Tractus later alis minor
Hypoglossal neme
Fig. 87. Lateral view of brain stem of the sheep.
forms one of the nuclei of this nerve (Fig. 111). This bundle of fibers and the
associated mass of gray matter are known as the spinal tract and nucleus of the
spinal tract of the trigeminal nerve.
The restiform body (corpus restiforme or inferior cerebellar peduncle) lies
between the lateral border of the fourth ventricle and the roots of the vagus
and glossopharyngeal nerves in the rostral part of the medulla oblongata (Figs.
87-89). There is no sharp line of demarcation between it and the more cau-
dally placed clava and cuneate tubercle. It is produced by a large strand of
nerve-fibers, which run along the lateral border of the fourth ventricle and then
turn dorsally into the cerebellum. These fibers serve to connect the medulla
ANATOMY OF THE PONS 123
oblongata and spinal cord on the one hand with the cerebellum on the other.
By a careful inspection of the surface of the medulla it is possible to recognize
the source of some of the fibers entering into the composition of the restiform
body. The ventral external arcuate fibers can be seen entering it after crossing
over the surface of the lateral area; and the dorsal spinocerebellar tract can also
be traced into it from a position dorsal to the caudal extremity of the olive.
At the point where the restiform body begins to turn dorsally toward the
cerebellum, it is partly encircled by an elongated transversely placed elevation
formed by the ventral and dorsal cochlear nuclei (Figs. 87, 88). This ridge is
continuous on the one hand with the cochlear nerve, and on the other with
several bundles of fibers which run medialward over the floor of the fourth
ventricle and are known as the stria medullares acusticce (Fig. 89). The cochlear
nuclei are more prominent in the sheep, while the medullary striae are best seen
in the human brain. Just caudal to this ridge there is sometimes seen another,
running more obliquely across the restiform body, which is an outlying portion
of the pons and has been described by Essick (1907) under the name corpus
pontobulbare.
Nerve Roots. From the surface of the medulla oblongata there emerge in
linear order along the posterior lateral sulcus a series of root filaments, which
continues the line of the dorsal roots of the spinal nerves. These are the root-
lets of the glossopharyngealj vagus and accessory nerves. But unlike the dorsal
roots, which are made up of afferent fibers, the spinal accessory nerve contains
efferent fibers, while the vagus and glossopharyngeal are mixed nerves. The
line of the ventral or motor roots of the spinal nerves is continued in the medulla
oblongata by the root filaments of the hypoglossal neroe, which is also composed
of motor fibers. The abducens, facial, and acoustic nerves make their exit along
the caudal border of the pons in the order named from within outward. The
abducens emerges between the pons and the pyramid, the acoustic far lateral-
ward in line with the restiform body, and the facial with its sensory root, the
nervus intermedius , near the acoustic nerve (Figs. 86-88).
THE ANATOMY OF THE PONS
The pons, which is differentiated from the ventral part of the metencephalon,
is interposed between the medulla oblongata and the cerebral peduncles and
lies ventral to the cerebellum. As seen from the ventral surface, it is formed
by a broad transverse band of nerve-fibers, which on either side become aggre-
gated into a large rounded strand, the brachium pontis or middle cerebellar
124
THE NERVOUS SYSTEM
peduncle, and finally enter the corresponding hemisphere of the cerebellum
(Figs. 83, 86). This transverse band of fibers, which gives the bridge-like
form from which this part derives its name, belongs to the basilar portion of
the pons and is superimposed upon a deeper dorsal portion that may be regarded
as a direct upward continuation of the medulla oblongata. The transverse
fibers form a part of the pathway connecting the cerebral hemispheres with the
opposite cerebellar hemispheres; and the size of the pons, therefore, varies with
Anterior limb of
internal capsule
Head of the can- '
date nucleus
Anterior commissure''
A nterior perforated . -
substance
Optic nerve '
Basis pedunculi'
Pons
Nervus jportio minor
trigeminus \portio major
Acoustic nerve
Facial nerve
Glossopharyngeal and vagus nerves
Olive
Hypoglossal nerve
Ventral external arcuate fibers
Pyramid
Ventral root N. cero. I
Anterior lateral sulcns
Ventral root N. cerv. II ~
rr~^^^---s Corona radiata
Tail of the caudate nucleus
Lenticulotha-
lamic part Posterior
Retrolenticular limb of
part internal
Sublenticular | capsule
part J
Thalamus
Medial geniculate body
-- Superior colliculus
" ^Inferior quadrigeminal brachium
"" Inferior colliculus
" - Trochlear nerve
- - Lateral lemniscus
- - Brachium conjunctivum
^~* Fila later alia pontis
,> Dentate nucleus
Restiform body
Brachium pontis
~~ ~~ Dorsal cochlear nuc.
~~ Corpus pontobulbare
" Restiform body
~ Tuberculum cinereum
* Accessory nerve
--'"Dorsal root N. cerv. II
Fig. 88. Lateral view of human brain stem.
the size of these other structures. It is instructive to compare the brains of
the shark, sheep, and man with this point in mind (Figs. 11, 84, 85).
The ventral surface of the pons is convex from above downward and from
side to side and rests upon the basilar portion of the occipital bone and upon
the dorsum sellae (Fig. 81). A groove along the median line, the basilar sulcus,
lodges the basilar artery (Fig. 86).
The trigeminal nerve emerges from the ventral surface of the pons far lateral-
ward at the point where its constituent transverse fibers are converging to form
THE FOURTH VENTRICLE 125
the brachium pontis. In fact, it is customary to take the exit of this nerve as
marking the point of junction of the pons with its brachium. The nerve has two
roots which lie close together: the larger is the sensory root, or portio major;
the smaller is the motor root, or portio minor (Fig. 88).
The posterior surface of the pons forms the rostral part of the floor of the
fourth ventricle, along the lateral borders of which there are two prominent
and rather large strands of nerve-fibers, the brachia conjunctiva (Figs. 88, 89).
The brachia conjunctiva or superior cerebellar peduncles lie under cover of
the cerebellum. As they emerge from the white centers of the cerebellar hemi-
spheres they curve rostrally and take up a position along the lateral border of
the fourth ventricle. They converge as they ascend and disappear from view
by sinking into the substance of the mesencephalon under cover of the inferior
quadrigeminal bodies. Each consists of fibers which connect the cerebellum
with the red nucleus, a large gray mass situated within the midbrain ventral to
the superior colliculus of the corpora quadrigemina. The interval between the
two brachia conjunctiva, where these form the lateral boundaries of the fourth
ventricle, is occupied by a thin lamina of white matter, the anterior medullary
velum (Fig. 85). This is stretched between the free dorsomedial borders of the
two brachia and forms the roof of the rostral portion of the ventricle. Caudally
it is continuous with the white center of the cerebellum. The fibers of the
trochlear nerves decussate in the anterior medullary velum and emerge from its
dorsal surface (Fig. 89). As they run through the velum they produce a raised
white line which extends transversely from one brachium to the other.
THE FOURTH VENTRICLE
The lozenge-shaped cavity of the rhombencephalon is known as the fourth
ventricle. It lies between the pons and medulla oblongata, ventrally, and the
cerebellum dorsally, and is continuous with the central canal of the closed por-
tion of the medulla, on the one hand, and with the cerebral aqueduct on the
other (Fig. 84). On each side a narrow curved prolongation of the cavity ex-
tends laterally on the dorsal surface of the restiform body. This is known as
the lateral recess (Figs. 89, 90). It opens into the subarachnoid space near the
flocculus of the cerebellum; and through this lateral aperture of the fourth ven-
tricle (foramen of Luschka) protrudes a small portion of the chorioid plexus
(Fig. 90). There is also a median aperture (foramen of Magendie) through the
roof of the ventricle near the caudal extremity. By means of these three open-
ings, one medial and two lateral, the cavity of the ventricle is in communica-
126
THE NERVOUS SYSTEM
tion with the subarachnoid space, and cerebrospinal fluid may escape from the
former into the latter.
The floor of the fourth ventricle is known as the rhomboid fossa and is formed
by the dorsal surfaces of the pons and open part of the medulla oblongata, which
are continuous with each other without any line of demarcation and are irreg-
ularly concave from side to side (Figs. 89, 91). The fossa is widest opposite the
points where the restiform bodies turn dorsally into the cerebellum; and it
gradually narrows toward its rostral and caudal angles. The lateral boundaries
Pineal body
~ Superior colliculus
Inferior colliculus
Cerebral peduncle
Trochlear nerve
Median sulcus
Locus caruleus
Facial colliculus
Medial eminence
Sulcus limitans
Lateral recess
Stri(B medtillares
Tcenia
Trigonum hypoglossi
Cuneate tubercle
Tuber culum cinereum
Clava
Posterior median fissure
Posterior intermediate
sulcus
'Posterior lateral sulcus
Medial geniculate body--*-
Inferior quadrigeminal T~
brachium
Frenulum veli
A nterior medullary velum
Brachium conjunctivum
Brachium pontis*--^
Restiform body-
t
Superior fovea
Area acustica-<-=:~
Inferior fovea
Restiform body
Ala cinerea """
Funiculus separans"'"
Area postrema-"'"
Obex-''"'
Funiculus gracilis -~~~
Funiculus cuneatus~ "
Fig. 89. Dorsal view of human brain stem.
of the fossa, which are raised some distance above the level of the floor, are
formed by the following structures: the brachia conjunctiva, restiform bodies,
cuneate tubercles, and clava. Of the four angles to the rhomboid fossa, two
are laterally placed and correspond to the lateral recesses. At its caudal angle
the ventricle is continuous with the central canal of the closed part of the me-
dulla oblongata, and at its rostral angle with the cerebral aqueduct. Joining
the two last named angles there is a median sulcus which divides the fossa into
two symmetric lateral halves.
The rhomboid fossa is arbitrarily divided into three parts. The superior
THE FOURTH VENTRICLE
127
part is triangular, with its apex directed rostrally and its base along an imagin-
ary line through the superior foveae. The inferior part is also triangular, but
with its apex directed caudally and its base at the level of the horizontal por-
tions of the taeniae of the ventricle. Between these two triangular portions is
the intermediate part of the fossa, which is prolonged outward into the lateral
recesses. The floor is covered with a thin lamina of gray matter continuous
with that which lines the central canal and cerebral aqueduct. Crossing the
fossa transversely in its intermediate portion are several strands of fibers known
as the stria medullares acustica. These are subject to considerable variation in
different specimens. Springing from the dorsal cochlear nuclei they wind
around the restiform body in the lateral recess and run transversely across the
fossa to disappear in the median sulcus.
The inferior portion of the fossa bears some resemblance to the point of a
pen and has been called the calamus scriptorius. It belongs to the medulla
oblongata. In this part of the fossa there is on either side a small depression,
the inferior fovea, shaped like an arrow-head, the point of which is directed toward
the striae medullares. From the basal angles of this triangle run diverging sulci:
a medial groove toward the opening of the central canal and a lateral groove
more nearly parallel to the median sulcus. By these sulci the inferior portion
of the fossa is divided into three triangular areas. Of these the most medial
is called the trigone of the hypoglossal nerve or trigonum nervi hypoglossi. Be-
neath the medial part of this slightly elevated area is located the nucleus of the
hypoglossal nerve. The area between the two sulci, which diverge from the
fovea inferior, is the ala cinerea or triangle of the vagus nerve. Both names
are appropriate, the one, because of its gray color, and the other, because a
nucleus of the vagus nerve lies subjacent to it. The third triangular field,
placed more laterally, forms a part of the area acustica.
The area acustica is, however, not restricted to the inferior portion of the
fossa, but extends into the intermediate part as well. Here it forms a prominent
elevation over which the striae medullares run. Subjacent to this area lie the
nuclei of the vestibular nerve. A part of the acoustic area and all of the ven-
tricular floor rostral to it belong to the pons.
Rostral to the striae medullares there may be seen a shallow depression,
the fovea superior, medial to which there is a rounded elevation, the facial
colliculus. Under cover of this eminence the fibers of the facial nerve bend
around the abducens nucleus. Extending from the fovea superior to the
cerebral aqueduct is a shallow groove, usually faint blue in color, the locus
128
THE NERVOUS SYSTEM
caruleus, beneath which lies the substantia ferruginea, composed of pigmented
nerve-cells.
Beginning at the cerebral aqueduct and extending through both the superior
and inferior foveae is a very important groove, the sulcus limitans, which repre-
sents the line of separation between the parts derived from the alar plate and
those which originate from the basal plate of the embryonic rhombencephalon.
Lateral to this sulcus lie the sensory areas of the ventricular floor, including the
area acustica, all of which are derived from the alar plate. Medial to this
sulcus there is a prominent longitudinal elevation, known as the medial eminence,
which includes two structures already described, namely, the facial colliculus
and the trigone of the hypoglossal nerve. Beneath the medial part of this
Tel a chorioidea
Choriold plexus
Median aperture of
fourth ventricle
Fig. 90. Dorsal view of human rhombencephalon showing tela chorioidea and chorioid plexus of
the fourth ventricle.
trigone lies the nucleus of the hypoglossal nerve and beneath the lateral part is a
group of cells designated as the nucleus intercalatus.
One or two features remain to be mentioned. At the caudal end of the ala
cinerea is a narrow translucent obliquely placed ridge of thickened ependyma,
known as the funiculus separans. Between this ridge and the clava is a small
strip of the ventricular floor, called the area postrema, which on microscopic
examination is found to be rich in blood-vessels and neurogliar tissue.
The roof of the fourth ventricle is formed by the anterior medullary velum,
a small part of the white substance of the cerebellum, and by the tela chorioidea
lined internally by ependymal epithelium (Fig. 85). Caudal to the cerebellum
the true roof of the cavity is very thin and consists only of a layer of ependymal
epithelium, which is continuous with that lining the other walls of the ventricle.
ANATOMY OF THE MESENCEPHALON I2Q
This is supported on its outer surface by a layer of pia mater, the tela chorioidea,
rich in blood-vessels. From this layer vascular tufts, covered by epithelium,
are invaginated into the cavity and form the chorioid plexus of the fourth ven-
tricle (Fig. 90). The plexus is invaginated along two vertical lines close to the
median plane and along two horizontal lines, which diverge at right angles from
the vertical ones and run toward the lateral recesses. These right and left
halves are joined together at the angles so that the entire plexus has the shape
of the letter T, the vertical limb of which, however, is double.
After the tela chorioidea with its epithelial lining has been torn away to
expose the floor of the ventricle, there remains attached to the lateral bound-
aries of the caudal part of the cavity the torn edges of this portion of the roof.
These appear as lines, the teenies of the fourth ventricle, which meet over the
caudal angle of the cavity in a thin triangular lamina, the obex (Fig. 89). Ros-
trally each taenia turns lateralward over the restiform body and forms the caudal
boundary of the corresponding lateral recess.
THE MESENCEPHALON
The midbrain or mesencephalon occupies the notch in the tentorium and
connects the rhombencephalon, on the one side of that shelf-like process of
dura, with the prosencephalon on the other (Fig. 81). It consists of a. dorsal
part, the corpora quadrigemina, and a larger ventral portion, the cerebral pe-
duncles. It is tunneled by a canal of relatively small caliber, called the cerebral
aqueduct, which connects the third and fourth ventricles and is placed nearer
the dorsal than the ventral aspect of the midbrain (Fig. 84).
The cerebral peduncles (pedunculi cerebri, crura cerebri), as seen on the
ventral aspect of the brain, diverge like a pair of legs from the rostral border of
the pons (Fig. 83). Just before they disappear from view by entering the ven-
tral surface of the prosencephalon they enclose between them parts of the hypo-
thalamus, and are encircled by the optic tracts. On section, each peduncle is
seen to be composed of a dorsal part, the tegmentum, and a ventral part, the
basis pedunculi. Between the basis pedunculi and the tegmentum there inter-
venes a strip of darker color, the substantia nigra (Fig. 113). By dissection it is
easy to show that the basis pedunculi is composed of longitudinally coursing
fibers which can be traced rostrally to the internal capsule (Fig. 88). In the
other direction some of these fibers can be followed into the corresponding pyra-
mid of the medulla oblongata. On the surface two longitudinal sulci mark the
plane of separation between the tegmentum and the basis pedunculi. The
130
THE NERVOUS SYSTEM
groove on the medial aspect of the peduncle, through which emerge the fibers
of the third nerve, is known as the sulcus of the oculomotor nerve, while that on
the lateral aspect is called the lateral sulcus of the mesencephalon. Dorsal to
this latter groove the tegmentum comes to the surface and is faintly marked by
fine bundles of fibers which curve dorsally toward the inferior colliculus of the
corpora quadrigemina (Fig. 88). These fibers belong to the lateral lemniscus,
the central tract associated with the cochlear nerve.
The corpora quadrigemina form the dorsal portion of the mesencephalon,
and consist of four rounded eminences, the quadrigeminal bodies or colliculi,
Anterior limb of internal capsule.^
Stria terminalis^ /
Habenular commissure
Habenular trigone. \
Pineal body^ /\R
Posterior limb of internal capsule^. )\l"
Superior colliculus X"<-
Optic radiation % ?\ v ^
Attachment anterior'^!
medullary velum -J
Inferior colliculus
Superior fovea . ^
Brachium conjunctivum ^v\
O
Brachium pontis
Restiform body
Dorsal cochlear nucleus.
Acoustic area
Inferior fovea and restiform body ~-~
Tcenia of fourth ventricle
Clava
Cuneale tubercle
Posterior lateral sulcus
Corona radiata
-Head of cattdate nucleus
.Stria medullaris of thalamus
_ .- Third ventricle
, Thalamus
'\ ,, Tail of caudate nucleus
\, Median sulcus
W . Trochlear nerve
/''/Facial colliculus
' ,/,' Trigeminal nerve
'/Sulcus limitans
^'Medial eminence
Ala cinerea
.. Lateral recess of fourth ventricle
Trigone of hypoglossal nerve
Obex
'--Posterior median fissure
Posterior intermediate sulcus
Funiculus gracilis
Funiculus cuneatus
Fig. 91. Dorsal view of brain stem of sheep.
which arise from the dorsal aspect of a plate of mingled gray and white matter
known as the quadrigeminal lamina (Figs. 89, 91). The superior colliculi are
larger than the inferior, the disproportion being greater in the sheep than in
man. A median longitudinal groove separates the colliculi on either side. In
the rostral end of this groove rests the pineal body, while attached to its caudal
end is a band which runs to the anterior medullary velum, and is known as the
frenulum veli. A transverse groove runs between the superior and inferior collic-
uli and extends on to the lateral aspect of the mesencephalon, where it inter-
venes between the superior colliculus and the inferior quadrigeminal brachium
(Figs. 87, 89).
ANATOMY OF THE MESENCEPHALON 131
The Brachia of the Corpora Quadrigemina. From each colliculus there runs
ventrally and rostrally on the lateral aspect of the mesencephalon an arm or
brachium (Figs. 87, 88). The inferior quadrigeminal brachium is the more con-
spicuous and is the only one that can be readily identified in the sheep. It
runs from the inferior colliculus to the medial geniculate body. This is an oval
eminence, belonging to the diencephalon, which has been displaced caudally so
as to lie on the lateral aspect of the mesencephalon. The superior quadrigeminal
brachium runs from the superior colliculus toward the lateral geniculate body,
passing between the pulvinar of the thalamus and the medial geniculate body.
Some of the fibers can be traced beyond the lateral geniculate body into the
optic tract.
CHAPTER IX
THE STRUCTURE OF THE MEDULLA OBLONGATA
THE medulla oblongata contains the nerve-cells and fiber tracts associated
with certain of the cranial nerves. These include the central mechanisms which
control the reflex activities of the tongue, pharynx, and larynx, and in part those
of the thoracic and abdominal viscera also. At the same time the ascending
and descending fiber tracts, which unite the spinal cord with higher nerve
centers, pass through the medulla oblongata.
The central connections of the cranial nerves, except those of the first two
pairs, are located in the medulla oblongata and in the tegmental portions of the
pons and mesencephalon. In many respects they resemble the connections of
the spinal nerves within the spinal cord. The following general statements on
this topic, most of which are illustrated in Fig. 92, will help to elucidate the
structure of the brain stem.
1. The cells of origin of the sensory fibers of the cranial nerves (Fig. 92, 1)
are found in ganglia which lie outside the cerebrospinal axis and are homologous
with the spinal ganglia. These are the semilunar ganglion of the trigeminal,
the geniculate ganglion of the facial, the superior and petrous ganglia of the
glossopharyngeal, the jugular and nodose ganglia of the vagus, the spiral gang-
lion of the cochlear, and the vestibular ganglion of the vestibular nerve.
2. All of these sensory ganglia except the last two, the cells of which are
bipolar, are formed by unipolar cells, the axons of which divide dichotomously
into peripheral and central branches. The latter (or in the case of the acoustic
nerve the central processes of the bipolar cells) form the sensory nerve roots,
enter the brain stem and divide, each into a short ascending and a long descending
branch. These branches give off numerous collaterals, which with the terminal
branches end in gray masses known as sensory nuclei or nuclei of termination.
It is the descending branches of the sensory fibers of the trigeminal neroe which
form the spinal tract of that nerve illustrated in Figs. 92, 98, 99, 101.
3. The ascending branch may be entirely wanting, as in the case of the sen-
sory fibers of the seventh, ninth, and tenth nerves, all of which bend caudally and
form a descending tract in the medulla oblongata, known as the tractus soli-
tarius (Figs. 92, 101, 103).
132
THE STRUCTURE OF THE MEDULLA OBLONGATA
133
4. The sensory nuclei (Fig. 92, 4), within which the afferent fibers terminate,
contain the cells of origin of the sensory fibers of the second order (Fig. 92, 2).
Some of these are short; others are long, and these may be either direct or
crossed. Many of them divide into ascending and descending branches. They
run in the reticular formation and some of the ascending fibers reach the thal-
amus.
5. These sensory fibers of the second order give off collaterals to the motor
nuclei. Direct collaterals from the sensory fibers of the cranial nerves to the
motor nuclei are few in number or entirely wanting.
6. The motor nuclei (Fig. 92, 5) are aggregations of multipolar cells which
give origin to the motor fibers of the cranial nerves (Fig. 92, 3).
Main sensory nucleus
of trige minal nerve
Afferent fiber of
second order
Tractus solilarius
N2(deus of
hypoglossal nerve
Afferent fiber of
second order
Spinal tract of
trigeminal nerve
and Us nucleus
Fig. 92. Diagram of the tongue and rhombencephalon to illustrate the central connections
and functional relationships of certain of the cranial nerves: 1, Sensory neurons of the first order
of the trigeminal and glossopharyngeal nerves; 2, sensory neurons of the second order; 3, motor
fibers of the hypoglossal nerve; 4, sensory nuclei; 5, motor nucleus of hypoglossal nerve. (Cajal.)
The Rearrangement Within the Medulla Oblongata of the Structures Con-
tinued Upward from the Spinal Cord. At the level of the rostral border of the
first cervical nerve the spinal cord goes over without a sharp line of demarcation
into the medulla oblongata. The transition is gradual both as to external form
and internal structure; but in the caudal part of the medulla there occurs a
gradual rearrangement of the fiber tracts and alterations in the shape of the
gray matter, until at the level of the olive, a section of the medulla bears no
resemblance to one through the spinal cord.
The realignment of the corticospinal tracts and the termination of the fibers
of the posterior funiculi of the spinal cord are two of the most important factors
134
THE NERVOUS SYSTEM
responsible for this gradual transformation. Traced rostrally from the spinal
cord, the ventral corticospinal tracts are seen to enter the pyramids within the
ventral area of the medulla oblongata, that is to say, they enter the medulla
without realignment. But the fibers of the lateral corticospinal tracts on enter-
ing the medulla swing ventromedially in coarse bundles, which run through
the anterior gray columns and cut them off from the gray matter surrounding
the central canal (Figs. 93, 95). After crossing the median plane in the decussa-
tion of the pyramids these fibers join those of the opposite ventral corticospinal
tracts and form the pyramids (Fig. 96). Thus fibers from the lateral funiculus
come to lie ventral to the central canal and displace this dorsally; and at the same
time a start is made toward breaking up the H -shaped gray figure characteristic
of the spinal cord.
Cerebral hemisphere
Spinal
cord
Fig. 93. Diagram of the corticospinal tracts.
Shortly after entering the medulla oblongata the fibers of the posterior funiculi
end in nuclear masses which invade the funiculus gracilis and funiculus cuneatus
as expansions from the posterior gray columns and central mass of gray sub-
stance (Figs. 95, 96). These are known as the nucleus gracilis and nucleus cu-
neatus. They cause a considerable increase in the size of the posterior funiculi
and a corresponding ventrolateral displacement of the posterior columns of
gray matter. The fibers of the posterior funiculi end in these nuclei about cells,
the axons of which run ventromedially as the axis-cylinders of internal arcuate
fibers. These sweep in broad curves through the gray substance, and decus-
sate ventral to the central canal in what is known as the decussation of the medial
lemniscus. After crossing the median plane they turn rostrally between the
THE STRUCTURE OF THE MEDULLA OBLONGATA
135
pyramids and the central gray matter to form on either side of the median
plane a broad band of fibers known as the medial lemniscus (Figs. 96, 97).
Fasciculus gracilis -
Fasciculus cuneatus
Dorsolateralfasc. (Lissauer)
Substantial gelatinosa
Dorsal column
Lateral cortices pinal tract
Central canal "
Ventral column
Ventral corticospinal tract "
Funiculus gracilis
Nucleus gracilis
Funicnlus cuneatus
Spinal tract of trigem. nerve
Nucleus of spinal tract of
Dorsal column [N. V
Lateral corticospinal tract
Central canal
Decussation of the pyramids
Ventral column
Fig. 94.
Fig. 95.
Funiculus gracilis
Nucleus gracilis
Funiculus cuneatus
Nucleus cuneatus
Spinal tract of trigeminal nerve
Nucleus of spinal tract of N. V
^Central gray matter
Internal arcuate fibers
Central canal
Reticular substance
Medial lemniscus
Decussation of medial lemniscus
Decussation of the pyramids
Pyramid, corticospinal tract
Fig. 96.
- Fourth ventricle
..-Dorsal motor nucleus of vagus
Nucleus of hypoglossal nerve
Tractus solitarius
Nucleus of spinal tract of N. V
~t Spinal tract of trigeminal nerve
"Fibers of hypoglossal nerve
" Reticular substance
'Dorsal accessory olivary nucleus
"Medial lemniscus
'Inferior olivary nucleus
'Medial accessory olivary nucleus
Pyramid, ccrticospinal tract
nganr
Fig. 97.
Figs. 9497. Diagrammatic cross-sections to show the relation of the structures in the
medulla oblongata to those in the spinal cord: Fig. 94, First cervical segment of spinal cord;
Fig. 95, medulla oblongata, level of decussation of pyramids; Fig. 96, medulla oblongata, level
of decussation of medial lemniscus; Fig. 97, medulla oblongata, level of olive.
At the level of the middle of the olive most of the fibers of the funiculus cune-
atus and funiculus gracilis have terminated in their respective nuclei; and the
nuclei also disappear a short distance farther rostrally (Fig. 97). With the
136 THE NERVOUS SYSTEM
disappearance of these fibers and nuclei there ceases to be any nervous sub-
stance dorsal to the central canal, and this, which has been displaced dorsally
by the accumulation of the corticospinal fibers and those of the lemniscus ven-
tral to it, opens out as the floor of the fourth ventricle (Fig. 97).
The outline of the gray matter in the most caudal portions of the medulla
oblongata closely resembles that of the spinal cord. The anterior columns are
first cut off by the decussation of the pyramids (Fig. 95). Then the posterior
columns are displaced ventrolaterally due to the increased size of the posterior
funiculi and the disappearance of the lateral corticospinal tracts from their
ventral aspects. This rotation of the posterior column causes the apex of
that column with its spinal tract and nucleus of the trigeminal nerve, which are
continuous with the fasciculus dorsolateralis and substantia gelatinosa of the
spinal cord (Fig. 94), to lie almost directly lateral ward from the central canal
(Fig. 96). The shape of the gray figure is still further altered by the develop-
ment of special nuclear masses, many of which are very conspicuous. These
include the nucleus gracilis, nucleus cuneatus, inferior olivary nucleus, and the
nuclei of the cranial nerves. The greater part of the gray substance now becomes
broken up by nerve-fibers crossing in every direction, but especially by the
internal arcuate fibers. This mixture of gray and white matter is known as the
reticular substance. The central gray matter is pushed dorsad first by the pyra-
mids and later by the medial lemniscus until it finally spreads out to form a thin
gray covering for the floor of the fourth ventricle.
The Pyramids and Their Decussation. We have had occasion repeatedly
to refer to the crossing of the lateral corticospinal tracts in this and preceding
chapters, but there remain some details to be presented. The pyramids are
large, somewhat rounded fascicles of longitudinal fibers, which lie on either side
of the anterior median fissure 01 the medulla oblongata (Fig. 86). The constit-
uent fibers take origin from the large pyramidal cells of the anterior central
gyrus or motor cerebral cortex. The decussation of the pyramids or motor
decussation occurs near the caudal extremity of the medulla oblongata (Fig.
93). Approximately the medial three-fourths of the corticospinal tract passes
through the decussation into the lateral funiculus of the opposite side of the
spinal cord, as the lateral corticospinal tract (fasciculus cerebrospinalis lateralis
or lateral pyramidal tract); while the lateral one-fourth is continued without
crossing into the ventral funiculus of the same side as the ventral corticospinal
tract (fasciculus cerebrospinalis anterior or anterior pyramidal tract Figs.
94, 95, 96, 98). The decussating fibers are grouped into relatively large bundles
THE STRUCTURE OF THE MEDULLA OBLONGATA
137
as they cross the median plane, the bundles from one side alternating with
similar bundles from the other, and largely obliterating the anterior median fis-
sure at this level. There is great individual variation as to the relative size of
the ventral and lateral corticospinal tracts; and there may even be marked
asymmetry due to a difference in the proportion of the decussating fibers on the
two sides.
The nucleus gracilis and nucleus cuneatus (nucleus funiculi gracilis and
nucleus funiculi cuneati) are large masses of gray matter located in the pos-
terior funiculi of the caudal portion of the medulla oblongata. They are sur-
rounded by the fibers of these funiculi except on their ventral aspects, where they
are continuous with the remainder of the gray substance (Fig. 99). The fibers
Funiculus gracilis
Nucleus gracilis
Spinal tract of trigeminal
nerve
Nucleus of spinal tract of
N. V
Central canal
Decussation of the pyramids
Anterior column
Posterior median fissure
Funiculus cuneatus
Nucleus cuneatus
Dorsal spinocerebellar tract
Ventral spinocerebellar tract
Ventral fasciculus proprius
Bulbospinal tract
Anterior median fissure
Fig. 98. Section through the medulla oblongata of a child at the level of the decussation of the
pyramids. Pal-Weigert method. (X6.)
of the gracile and cuneate fasciculi terminate in the corresponding nuclei; and
their terminal arborizations are synaptically related to the neurons, whose cell
bodies and dendrites are located there (Fig. 100). Accordingly, in sections
through successive levels we see the fibers decreasing in number as the nuclei
grow larger (Figs. 98, 99, 101). It is due to the presence of these nuclei that the
funiculi become swollen to form the club-shaped prominences with which we are
already familiar under the names clava and cuneate tubercle. At the level of the
pyramidal decussation the gracile nucleus has the form of a rather thin and
ill-defined plate, while the cuneate nucleus is represented by a slight projection
from the dorsal surface of the posterior gray column (Fig. 98). At the level of
the decussation of the lemniscus both have enlarged and the gracile nucleus has
become sharply outlined (Fig. 99). As the central canal opens out into the
138
THE NERVOUS SYSTEM
fourth ventricle the nuclei are displaced laterally and gradually come to an end
as the restiform body becomes clearly defined (Fig. 101).
As one would expect from the fact that there is no sharp line of separation between the
spinal cord and medulla oblongata, some of the fibers of the cuneate fasciculus end in the
substantia gelatinosa (here known as the nucleus of the spinal tract of the trigeminal nerve)
and in the remnant of the head of the posterior gray column (Fig. 100). There are three
smaller gray masses within the funiculus cuneatus: (1) the external round nucleus, an iso-
lated portion of the substantia gelatinosa, near which it is situated; (2) the internal round
nucleus, more variable in position; and (3) the accessory or lateral cuneate nucleus superficial
to the main nuclear mass.
Funiculus gracilis
Nucleus gracilis
Spinal tract of trigeminal
nerve
Nucleus of spinal tract
of N. V
Dorsal motor nucleus of.
vagus
Nucleus of hypoglossal
nerve
Decussation of medial
lemniscus
Lateral reticular nucleus
Medial accessory olivary
nucleus
Ventral external arcuate
fibers
Funiculus cuneatus
Nucleus cuneatus
Central canal
Internal arcuate fibers
Reticular substance
Dorsal spinocerebellar
tract
Ventral spinocerebellar
tract
Ventral fasciculus
proprius
Hypoglossal nerve
Pyramid, corticospinal
tract
Fig. 99. Section through the medulla oblongata of a child at the level of the decussation of the
medial lemniscus. (Pal-Weigert method.) (X 6.)
The Medial Lemniscus and its Decussation. The great majority of fibers
which arise from the cells in the nucleus gracilis and nucleus cuneatus sweep
ventromedially in broad concentric curves around the central gray substance
toward the median raphe (Fig. 99). As has been stated on a preceding page,
these are known as internal arcuate fibers, and as they cross those from the
opposite side in the raphe they form the decussation of the lemniscus (decussatio
lemniscorum, sensory decussation). After crossing the median plane they turn
rostrally in the medial lemniscus (fillet), and end in the thalamus (Fig. 235).
These longitudinal fibers constitute a broad band which lies close to the median
raphe, medial to the inferior olivary nucleus, and dorsal to the pyramids (Figs.
96, 97). By the accession of additional internal arcuate fibers this band in-
creases in size and spreads out dorsally until at the level of the middle of the
olive it is separated from the gray matter of the ventricular floor only by the
THE STRUCTURE OF THE MEDULLA OBLONGATA
139
fibers of the fasciculus longitudinalis medialis and the tectospinal tract (Fig.
101). The decussation of the lemniscus begins at the upper border of the
decussation of the pyramids, where the sensory fibers are grouped into coarse
bundles arching around the central gray matter (Fig. 99), and extends as far as
do the gracile and cuneate nuclei, that is, to about the middle of the olive. In
sections through the lower half of the olive the internal arcuate fibers describe
broad curves through the reticular formation and their decussation occupies a
considerable ventrodorsal extent of the raphe (Fig. 101).
Nerve cell in the nucleus cuneatus
Ramification of fibers from the fasciculus cuneatus
Nucleus cuneatus
Fasciculus
cuneatus
Subslantia
gelatinosa
Fig. 100. From a transverse section through the medulla oblongata of a kitten, to illustrate
the termination of the fibers of the fasciculus cuneatus, and at a the beginning of the internal
arcuate fibers. (Combined from drawings by Cajal.)
The arcuate fibers of the medulla oblongata may be separated into two
groups: those which run through the reticular formation constitute the inter-
nal arcuate fibers; and those which run over the surface of the medulla, the
external arcuate fibers. The internal arcuate fibers are of at least three kinds:
(1) those described in the preceding paragraph, which arise in the gracile and
cuneate nuclei and form the medial lemniscus; (2) sensory fibers of the second
order, arising in the sensory nuclei of the cranial nerves; and (3) olivocerebellar.
fibers, which will be considered in another paragraph. Our knowledge of the
external arcuate fibers is less satisfactory. From the nuclei of the posterior funic-
140
THE NERVOUS SYSTEM
uli and perhaps also from these funiculi themselves a group of dorsal external
arcuate fibers make their way to the restiform body along the dorsal surface of
the medulla (Fig. 101). According to Cajal these fibers are well developed in
man, but absent in the cat and rabbit. The ventral external arcuate fibers are
said to include a certain number which arise in the lateral reticular and arcuate
nuclei and run dorsolaterally over the surface of the medulla to reach the
cerebellum by way of the restifrom body (Fig. 104). The arcuate nuclei are
small irregular patches of gray matter situated on the ventromedial aspect of
the pyramid and continuous rostrally with the nuclei pontis, with which they
Spinal vestibular
nucleus
Dorsal external
arcuate fibers
Tractus solitarius
and nucleus
Nucleus of
hypoglossal nerve
Internal arcuate
fibers
Dorsal spinocere-
bellar tract
Medial longitudinal
fasciculus
Ventral spinocere-
bellar tract
Tectospinal tract
Medial lemniscus
Inferior olivary
nucleus
Hilus of olivary
nucleus
Pyramid, cor ti co-
spinal tract
Fig. 101. Section through the medulla oblongata of a child at
method. (X 6.)
Dorsal motor
nucleus of vagus
Nucleus cuneatus
Restiform body
Spinal tract and
nucleus N. V
Nucleus ambiguus
Reticular substance
Lateral reticular
nucleus
edial accessory
olivary nucleus
nferior olivary
nucleus
Hypoglossal nerve
Ventral external
arcuate fibers
the level of the olive. Pal-Weigert
seem to be homologous (Figs. 101, 103). They probably receive fibers from the
cerebral cortex by way of the pyramidal tracts; and, if so, the external arcuate
fibers which arise from them are homologous with the transverse fibers of the
pons.
Although the facts stated above are pretty well established, only a small part of the
ventral external arcuate fibers are thus accounted for. The origin and course of the majority
of these fibers is still obscure. According to Cajal (1909) they arise from the nuclei of the
posterior funiculus, curve ventrally and medially over the surface of the medulla oblongata,
penetrate the pyramids or the anterior median fissure, cross in the median raphe, and join
the medial lemniscus of the opposite side. On the other hand, Edinger (1911) gives to
THE STRUCTURE OF THE MEDULLA OBLONGATA
141
them the name "tractus cerebello-tegmentalis bulbi," and believes that they descend from
the cerebellum by way of the restiform body, then arch ventrally over the surface of the
medulla, penetrate the pyramid or the anterior median fissure, and end in the reticular
formation of the opposite side (Fig. 153). According to Van Gehuchten (1904) some of the
ventral external arcuate fibers arise from cells in the reticular formation of the same and the
opposite side, and run through the restiform body to the cerebellum.
Olivary Nuclei. The oval prominence in the lateral area of the medulla,
known as the olive, is produced by the presence just beneath the surface of a
large gray mass, the inferior olivary nucleus, with which there are associated
Fig. 102. Diagram to illustrate the structure of the inferior olivary nucleus. (Cajal, Edinger.)
two accessory olivary nuclei. The inferior olivary nucleus is very conspicuous
in the sections of this part of the medulla (Fig. 101). It appears as a broad,
irregularly folded band of gray matter, curved in such a way as to enclose a
white core, which extends into the nucleus from the medial side through an
opening, known as the hilus. Considered as a whole this nucleus resembles a
crumpled leather purse, with an opening, the hilus, directed medially. Sec-
tions at either end of the nucleus do not include this Opening, and at these
points the central core of white matter is completely surrounded by the gray
lamina. The fibers which stream in and out of the hilus constitute the olivary
142
THE NERVOUS SYSTEM
peduncle. The two accessory olives are plates of gray substance, which in trans-
verse section appear as rods. The medial accessory olivary nucleus is placed be-
tween the hilus of the inferior olive and the medial lemniscus, while the dorsal
accessory olivary nucleus is located close to the dorsal aspect of the chief nuclear
mass.
Structure and Connections. The gray lamina of the inferior olivary nucleus
consists of neuroglia and many rounded nerve-cells beset with numerous short,
frequently branching dendrites, the axons of which run through the white core
of the nucleus and out at the hilus as olivocerebellar fibers (Fig. 102) . About
these cells there ramify the end branches of several varieties of afferent fibers,
the origin of which is not well understood. Some come from a tract, designated
Fourth ventricle
Principal vestibular nucleus
Spinal vestibular nucleus
Nucleus intercalate
Restiform body
Spinal tract and
nucleus N. V
Ponlobulbar body
Glossopharyngeal nerve
Nucleus ambiguus
Ventral spinocerebellar tract
Dorsal accessory olivary
nucleus
Hilus of olivary nucleus
Inferior olivary nucleus
Medial accessory olivary nucleus
Ventral external arcuate fibers
Tcenia of fourth ventricle
Nucleus of hypoglossal nerve
Dorsal motor nucleus of vagus
Tractus solitarius and
nucleus
Medial longitudinal
fasciculus
Reticular substance
Olivocerebellar fibers
Vagus nerve
Lateral reticular nucleus
Thalamo-olivary tract
Inferior olivary nucleus
Medial lemniscus
Hypoglossal nerve
Pyramid, corticospinal tract
Arcuate nucleus
Fig. 103. Section through the medulla oblongata of a child at the level of the restiform body.
Pal-Weigert method. (X4.)
as the thalamo-olivary fasciculus; but it is not certain that they have their
origin in the thalamus; quite possibly they come from some other gray mass
in that neighborhood. Another group of fibers, consisting chiefly of collaterals,
comes from the ventral funiculus of the spinal cord and may be regarded as
ascending sensory fibers (Cajal, 1909). These belong to the so-called spino-
olivary fasciculus.
Olivocerebellar Fibers. The axons from the cells of the inferior olivary
nucleus stream out of the hilus, cross the median plane, and either pass through
or around the opposite nucleus. Here they are joined by some uncrossed fibers
from the olivary nucleus of the same side. Thence they curve dorsally toward
the restiform body, passing through the spinal tract of the trigeminal nerve
THE STRUCTURE OF THE MEDULLA OBLONGATA
which becomes split up into several bundles (Fig. 103). They form an im-
portant group of internal arcuate fibers, which run through the restiform body
to the cerebellum and constitute the olivocerebellar tract (Fig. 104).
The restiform body or inferior cerebellar peduncle is a large and prominent
strand of fibers which gradually accumulate along the lateral border of the
caudal part of the fourth ventricle. It forms the floor of the lateral recess of
that cavity and then turns dorsally into the cerebellum (Figs. 88, 89, 103). It
is composed for the most part of two large and important fascicles: (1) the
-'Restiform body
- -Olivocerebellar tract
'Lateral reticular nucleus
Medulla oblongata <
Spinal cord
""Arcuate fibers from arcuate
nucleus
Dorsal external arcuate fibers
Dorsal spinocerebellar tract
Fig. 104. Diagram showing the fiber tracts which enter the restiform body from the medulla
oblongata.
olivocerebellar fibers, both direct and crossed, but chiefly from the inferior olivary
nucleus of the opposite side; and (2) the dorsal spinocerebellar tract, from the
nucleus dorsalis of the same side of the spinal cord (Fig. 104). In addition,
there are fibers in smaller number from other sources: (3) the dorsal external
arcuate fibers from the gracile and cuneate nuclei of the same side; and fibers
(4) from the arcuate nucleus, (5) from the lateral reticular nucleus, and possibly
also from other cells scattered through the reticular formation (Van Gehuchten,
1904).
144
THE NERVOUS SYSTEM
The dorsal spinocerebellar tract can readily be traced in serial sections of
the medulla because the large, heavily myelinated fibers of which it is composed
cause it to be deeply stained by the Weigert technic. It can be followed from
the spinal cord along the periphery of the medulla oblongata near the posterior
lateral sulcus. At first it lies ventral to the spinal tract of the trigeminal nerve
(Figs. 98, 99). But at the level of the lower part of the olive it inclines dorsally,
passing over the surface of the spinal tract of this nerve to reach the restiform
body (Fig. 101). Between this tract and the olive we find the ventral spino-
cerebellar tract also in a superficial position.
The spinal tract of the trigeminal nerve is formed by the descending branches
of the sensory fibers of that nerve. They give off collateral and terminal
branches to a column of gray matter, resembling the substantia gelatinosa
Tractus solitarius and nucleus
Dorsal motor nucleus of vagus
Nucleus of hypoglossal nerve
Nucleus amblguus
Medial longitudinal fasciculus
Tectospinal tract
Dorsal accessory olivary nucleus
Medial lemniscus
Medial accessory olivary nucleus
Corticospinal tract
Fig. 105. Diagram showing the location of the nuclei
at the level of the
' Vestibular nudeus
.--Nucleus cuneatus
of the spinal tract N. V
'" Dorsal spinocerebellar tract
-- Spinal tract N. V
Vagus nerve
spinocerebellar tract
" Spinothalamic tract
Thalamo-olivary tract
-Inferior olivary nucleus
Hypoglossal nerve
and fiber tracts of the medulla oblongata
olive.
Rolandi, with which it is directly continuous, and designated as the nucleus
of the spinal tract of the trigeminal nerve (Figs. 92, 98, 99, 101, 103). The tract
lies along the lateral side of the nucleus and is superficial except in so far as it
is covered by the external arcuate fibers, the dorsal spinocerebellar tract, and the
restiform body. It forms an elongated elevation, the tuberculum cinereum on
the surface of the medulla oblongata (Fig. 88).
The formatio reticularis fills the interspaces among the larger fiber tracts
and nuclei. It is composed of small islands of gray matter, separated by' fine
bundles of nerve-fibers which run in every direction, but which are for the
most part either longitudinal or transverse. It is subdivided into two parts.
The formatio reticularis alba is located dorsal to the pyramid and medial to the
root filaments of the hypoglossal nerve and is composed in large part of longi-
THE STRUCTURE OF THE MEDULLA OBLONGATA 145
tudinal nerve-fibers belonging to the medial lemniscus, tectospinal tract, and the
medial longitudinal fasciculus (Fig. 105). The latter is closely associated with
the vestibular nerve and can best be described with the central connections of
that nerve. Theformatio reticularis grisea is found dorsal to the olive and lateral
to the hypoglossal nerve. In it the nerve-cells predominate and the trans-
versely coursing internal arcuate fibers form a conspicuous feature. Its longi-
tudinal fibers, though less prominent, are of great importance. The descend-
ing fibers include those of the rubrospinal tract, which can be followed into the
lateral funiculus of the spinal cord, and the thalamo-olivary fasciculus, which
ends in the olive. Among the ascending filers are those of the ventral and
dorsal spinocerebellor, the spinothalamic , and spinotectal tracts.
The neme-cells of the reticular formation are scattered through the mesh of
interlacing fibers. In certain localities they are more closely grouped and form
fairly well-defined nuclei. Among these we may select two for special atten-
tion. The lateral reticular nucleus or nucleus of the lateral funiculus is a long
column of cells found along the deep surface of the ventral spinocerebellar tract,
from which it is said by Andre Thomas to receive afferent fibers. At any rate,
it receives fibers from the lateral funiculus of the spinal cord (Cajal, 1909) and
sends its axons to the cerebellum by way of the restiform body (Van Gehuchten,
1904; Yagita, 1906). It seems, therefore, to be a way station on a sensory
path from the spinal cord to the cerebellum. Some large cells in the gray part
of the reticular formation may be grouped together and called the motor nucleus
of the tegmentum (nucleus magnocellularis of Cajal). Their axons become as-
cending or descending fibers or may bifurcate into ascending and descending
branches within the reticular formation. Kohnstamm has traced such fibers
by means of the degeneration method, and has shown that they run for the
most part in a caudal direction and that some of them reach the cervical por-
tion of the spinal cord (tractus reticulospinalis Fig. 115).
The nuclei of the cranial nerves can best be considered in a separate chapter.
At this point it will only be necessary to enumerate and locate the nuclei of those
nerves which take origin from the medulla oblongata.
The nucleus of the hypoglossal nerve contains the cells of origin of the
motor fibers which compose that nerve. It forms a long column of nerve-cells
on either side of the median plane in the ventral part of the gray matter sur-
rounding the central canal and in the floor of the fourth ventricle (Figs. 99, 101,
103). In the latter region it lies immediately beneath that part of the floor
which was described in the preceding chapter under the name of the trigonum
146 THE NERVOUS SYSTEM
hypoglossi (Fig. 89). In reality, it corresponds only to the medial part of this
eminence, for on its lateral side there is found another group of cells known as
the nucleus intercalatus (Fig. 103). From their cells of origin the fibers of
the hypoglossal nerve stream forward through the reticular formation to emerge
at the lateral border of the pyramid.
The nucleus ambiguus is a long column of nerve-cells which give origin to
the motor fibers that run through the glossopharyngeal, vagus, and accessory
nerves to supply the striated musculature of the pharynx and larynx. It is
located in the reticular formation of both the open and the closed portions
of the medulla, ventromedial to the nucleus of the spinal tract of the trigeminal
nerve (Figs. 101, 103).
The dorsal motor nucleus of the vagus lies along the lateral side of the
nucleus of the hypoglossal. It occupies the ala cinerea of the rhomboid fossa
and extends into the closed part of the medulla oblongata along the lateral
side of the central canal (Figs. 89, 99, 101, 103). From the cells of this nucleus
arise the efferent fibers of the vagus nerve which innervate smooth muscle
and glandular tissue. The afferent fibers of the vagus and glossopharyngeal
nerves bend caudally and run within the tractus solitarius.
The nucleus of the tractus solitarius is the nucleus of reception of the affer-
ent fibers of the facial, glossopharyngeal, and vagus nerves, i. e., it contains
the cells about which these afferent fibers terminate. The tractus solitarius
can be traced throughout almost the entire length of the medulla. It decreases
in size as the descending fibers terminate in the gray matter which surrounds
it (Figs. 92, 101, 103).
CHAPTER X
INTERNAL STRUCTURE OF THE PONS
THE pons consists of two portions which differ greatly in structure and sig-
nificance. The dorsal or tegmental part resembles the medulla oblongata, of
which it is the direct continuation. The ventral or basilar portion contains
the longitudinal fibers which go to form the pyramids; but except for these it is
composed of structures which are peculiar to this level. It is a recent phyletic
development and forms a prominent feature of the brain only in those mam-
mals which have relatively large cerebral and cerebellar hemispheres, as might
be expected from the fact that it forms part of a conduction path uniting these
structures.
THE BASILAR PART OF THE PONS
The basilar portion of the pons is the larger of the two divisions. It is
made up of fascicles of longitudinal and transverse fibers and of irregular masses
of gray substance, which occupy the spaces left among the bundles of nerve-
fibers and which are known as the nuclei pontis.
The longitudinal fasciculi of the pons consist of two kinds of fibers: (1) those
of the corticospinal tract, which are continued through the pons into the pyra-
mids of the medulla oblongata; and (2) those which end in the nuclei of the pons
and are known as corticopontine fibers (Fig. 106). As they pass through the pons
the corticospinal fibers give off collaterals which also end in these nuclei. The
longitudinal fibers enter the pons at its rostral border from the basis pedunculi.
At first they form on either side a single compact bundle; but this soon becomes
broken up into many smaller fascicles, which are separated from each other
by the transverse fibers and nuclei of the pons (Fig. 108). At the caudal border
these bundles again become assembled into a compact strand, which is con-
tinued as the pyramid of the medulla oblongata (Fig. 107). It is evident, how-
ever, that the volume of the bundles is much greater at the rostral than at the
caudal border. This is to be explained by the fact that the corticopontine
fibers have left these bundles during their passage through the pons and have
come to an end by arborization within the nuclei pontis.
The transverse fibers are designated as fibres pontis and are divisable into a
superficial and a deep group (fibrae pontis superficiales and fibrae pontis pro-
147
148
THE NERVOUS SYSTEM
funda). Those of the superficial group lie ventral to the longitudinal fasciculi;
while the deep transverse bundles interlace with the longitudinal ones or lie
dorsal to them. The majority of the fibrae pontis cross the median plane. These
are joined by some uncrossed fibers and gathered together on either side of the
pons to form a compact and massive strand, known as the brachium pontis or
middle cerebellar peduncle, which curves dorsally to enter the white center of
the cerebellum (Figs. 88, 108).
^X * >v
V Cerebral cortex,
-y Corticobulbar tract
- * Corticospinal tract
Temporopontine tract
Frontopontine tract
- Pons
- Cerebellum
"' Nuclei pontis
x Brachium pontis
Lateral corticospinal tract
Ventral corticospinal tract
Fig. 106. Diagram of the cortico-ponto-cerebellar pathway and the corticospinal and cortico-
bulbar tracts.
Along the rostral border of the pons and brachium pontis one or two fiber bundles are
sometimes found which run an isolated course to the cerebellum. These are known as the
fila later alia pontis or Icenia pontis (Fig. 88). According to Horsley (1906) the constituent
fibers arise from a ganglion situated caudal to the interpeduncular ganglion, decussate at once,
and end in the cerebellum in the neighborhood of the dentate nucleus. Perhaps they rep-
resent slightly displaced fibrae pontis. Some of the transverse fibers on reaching the median
plane bend at right angles and run as fibrse rectae toward the pars dorsalis pontis (Fig. 108).
According to Edinger (1911) these belong in part at least to the tractus cerebellotegmentalis
pontis, which arises in the nuclei of the cerebellum and runs through the brachium pontis
to end in the reticular formation of the opposite side (Fig. 153). Cajal (1909) is doubtful
about the existence of such efferent fibers from the cerebellum in the brachium pontis.
The nuclei pontis, which are continuous with the arcuate nuclei of the
medulla oblongata, contain stellate nerve-cells of varying size, the axons of
INTERNAL STRUCTURE OF THE PONS 149
which are continuous with the fibrse pontis. There are also some small nerve-
cells of Golgi's Type II, the short axons of which end in the adjacent gray mat-
ter. Within these nuclei terminate the fibers of the corticopontine tracts and
some collaterals from the corticospinal fibers. Collaterals from the medial
lemniscus are also found arborizing in those nuclei of the pons which lie im-
mediately ventral to that bundle. This gray matter, therefore, represents an
important association apparatus within which there terminate fibers from
several different sources.
From what has been said it will be apparent that the pons serves to estab-
lish an important and for the most part crossed connection between the cere-
bral hemispheres and the cerebellum, a cortico-ponto-cerebellar path. The cor-
ticopontine fibers take origin from pyramidal cells in the frontal and temporal
lobes and end in the nuclei pontis. Arising from the cells in these nuclei, most
of the transverse fibers cross the median plane and reach the opposite cerebellar
hemisphere through the brachium pontis (Fig. 106).
THE DORSAL OR TEGMENTAL PART OF THE PONS
The dorsal or tegmental part of the pons (pars dorsalis pontis) resembles in
structure the medulla oblongata (Fig. 108). On its dorsal surface there is a
thick layer of gray matter which lines the rhomboid fossa. Between this layer
and the basilar portion of the pons is the reticular formation divided by the
median raphe into two symmetric halves. This has essentially the same struc-
ture here as in the medulla oblongata, and contains the continuation of many
longitudinal tracts with which we are already familiar. The restiform body at first
occupies a position similar to that which it has in the medulla, along the lateral
border of the rhomboid fossa; but it soon bends dorsally into the cerebellum.
The Cochlear Nuclei. At the point of transition between the medulla and
pons the restiform body is partly encircled on its lateral aspect by a mass of
gray matter formed by the terminal nuclei of the cochlear division of the acoustic
nerve (Fig. 107). There may be distinguished a dorsal and a -ventral cochlear
nucleus at the dorsal and ventral borders of the restiform body. Within these
nuclei the fibers of the cochlear nerve end; while those of the vestibular nerve
plunge into the substance of the pons ventromedially to the restiform body to
reach the floor of the fourth ventricle (Fig. 134). Fibers from the dorsal cochlear
nucleus run medially upon the floor of the fourth ventricle in the striae medullares
(Fig. 89), and sinking into the tegmentum join the fibers from the ventral coch-
lear nucleus in the trapezoid body.
THE NERVOUS SYSTEM
The trapezoid body (corpus trapezoideum) , which in most mammals appears
on the surface of the medulla near the border of the pons (Fig. 83), is covered
in man by the enlarged pars basalis pontis. In sections through the more caudal
portions of the pons the trapezoid body forms a conspicuous bundle of trans-
verse fibers in the ventral portion of the reticular formation (Fig. 108). The
fibers are associated with the terminal nuclei of the cochlear nerve, especially
the ventral one, and with the superior olivary nucleus, around the ventral border
of which they swing in such a way as to form a bay for its reception. Farther
medialward they pass through the medial lemniscus at right angles to its con-
Fourth ventricle
Stria medullares
Dorsal cochlear nucleus
Vent, spinocerebellar tract
Vent, external arcuate fibers
Medial lemniscus
Nucleus of eminentia teres
Principal vestibular nucleus
Lateral vestibular
nucleus
Nucleus of tract us
solitarius
Glossopharyngcal
nerve
Dorsal cochlear
nucleus
Restiform body
Ventral cochlear
nucleus
Spinal tract and
nucleus N. V
Trapezoid body
Pontobulbar body
Medial longitudinal fasciculus
Thalamo-olivary tract
Inferior olivary nucleus
Pyramid, corticospinal tract
Arcuate nucleus
Foramen cacum Pons
Fig. 107. Section through caudal border of the pons and the cochlear nuclei of a child.
Weigert method. ( X 4.)
Pal-
stituent fibers and decussate in the median raphe. The trapezoid body de-
scribes a curve with convexity directed rostrally as well as ventrally, and as a
result its lateral portions are seen best in sections through the lower border
of the pons (Fig. 107), while the rest of it is in evidence in sections at a higher
level (Fig. 108). Arising from the ventral nucleus of the cochlear nerve (Fig.
107) these fibers pass, with or without interruption in the superior olivary
nucleus, across the median plane (Fig. 108) ; and, on reaching the lateral border
of the opposite superior olivary nucleus, they turn rostrally to form a longi-
tudinal band of fibers known as the lateral lemniscus (Fig. 110). This is a
INTERNAL STRUCTURE OF THE PONS
part of the central auditory pathway, the connections of which are represented
diagrammatically in Fig. 134.
The superior olivary nucleus is a small mass of gray matter located in the
ventrolateral portion of the reticular formation of the pons in close relation to
the trapezoid body and not far from the rostral pole of the inferior olivary nucleus
(Figs. 108, 110). It consists of two or three separate but closely associated
nuclear masses composed of small fusiform nerve-cells, among which there
ramify collaterals from the fibers of the trapezoid body. From the dorsal aspect
Superior vestibular nucleus
Abducens nerve
Genu of facial N. /
Medial longitudinal
fasciculus
Fourth ventricle
Restiform body
Brachium pontis
Nucleus of abducens N.
Facial nerve
Spinal tract and nu-
cleus N. V
Nucleus of facial N.
Thalamo-olivary tract
Superior olivary nucleus
Trapezoid body and
medial lemniscus
Deep stratum of pons
Corticospinal and cortico-
pontine tracts
Nuclei pontis
'ficial stratum of pons
Fig. 108. Section through the pons of a child at the level of the facial colliculus. Pal-Weigert
method. (X 4.)
of this nucleus a bundle of fibers, known as the peduncle of the superior olive,
makes its way toward the nucleus of the abducens nerve, and it may be that
some of these fibers enter the medial longitudinal bundle (Fig. 124).
The nuclei of the vestibular nerve lie in the floor of the fourth ventricle,
where they occupy a field with which we are already familiar, namely, the area
acustica (Fig. 89). The vestibular fibers on approaching the rhomboid fossa
divide into ascending and descending branches, and terminate in four nuclear
masses: (1) the medial (dorsal or principal) vestibular nucleus (Figs. 103, 107),
(2) the lateral vestibular nucleus of Deiters (Fig. 107) , (3) the superior vestibular
152
THE NERVOUS SYSTEM
nucleus of Bechterew (Fig. 108), (4) the spinal or descending vestibular nucleus
(Fig. 103). These are represented diagrammatically in Fig. 136.
The medial longitudinal fasciculus is an important bundle which extends
from near the floor of the third ventricle to the spinal cord, and is especially
concerned with the reflex control of the movements of the head and eyes. A
large proportion of its fibers are derived from the lateral vestibular nucleus.
~M. rectus medialis
'i'\M. rectus lateralis
Nucleus of med. long. fasc.
Nucleus of oculomotor nerve
Nucleus of trochlear nerve
Nucleus of abducens nerve
Medial longitudinal fasciculus
r Lateral vestibular nucleus
Vestibular nerve
Fig. 109. Diagram showing the connections of the medial longitudinal fasciculus. (Modified
from Villiger.)
From this origin the fibers pass horizontally through the reticular formation to
the median longitudinal fasciculus of the same or the opposite side, and there
divide into ascending and descending branches (Fig. 109) . The former terminate
in the nuclei of the oculomotor, trochlear, and abducens nerve, the latter in
the nucleus of the spinal accessory nerve and in the columna anterior of the
cervical portion of the spinal cord. In this way there is established a path for
INTERNAL STRUCTURE OF THE PONS 153
the reflex control of the movement of the head, neck, and eyes in response to
stimulation of the nerve endings in the semicircular canals of the ears. Another
important group of fibers within this fasciculus takes origin from a collection
of cells situated in the hypothajamus just rostral to the red nucleus, which
Cajal (1911) has called the interstitial nucleus, 1 but which might properly be
designated as the nucleus of the medial longitudinal fasciculus. According to
Cajal the fascicle also contains ascending fibers from the ventral fasciculus
proprius of the spinal cord. Still other fibers serve to connect the nuclei of the
oculomotor and abducens nerves.
The medial longitudinal fasciculus is continued into the ventral fasciculus
proprius of the spinal cord. These fibers are displaced dorsolaterally by the
decussation of the pyramids (Fig. 98) and then still farther dorsally by the
decussation of the lemniscus (Fig. 99) until they come to lie in the most dorsal
part of the substantia reticularis alba (Fig. 101), which position they occupy
throughout the remainder of their course. The fasciculus is found ventral to
the nucleus of the hypoglossal nerve (Fig. 103) and in close apposition to the
nuclei of the three motor nerves of the eye (Figs. 108, 114, 116).
The medial lemniscus can also be traced within the reticular formation from
the medulla into and through the pons. But this broad band of longitudinal
fibers, which was spread out along the median raphe in the medulla, shifts
ventrally in the pons, assuming first a somewhat triangular outline and a ven-
tromedian position (Fig. 107); then by shifting farther lateralward it takes
again the form of a flat band (Figs. 108, 110). But now it is compressed ven-
trodorsally and occupies the ventral part of the reticular formation, its fibers
crossing those of the trapezoid body at right angles. It must not be forgotten
that the medial lemniscus is composed of longitudinal fibers, and it is by the
gradual shifting of these that the bundle as a whole changes shape and posi-
tion. As it is displaced ventrally it separates from the medial longitudinal
bundle, which retains its dorsal position.
The motor nucleus of the facial nerve occupies a position in the reticular
formation dorsal to the superior olive (Fig. 108). It is an oval mass of gray
matter, which extends from the lower border of the pons to the level of the
facial colliculus, and contains the cells of origin of the fibers which innervate
1 The interstitial nucleus of Cajal must not be confused with the nucleus of the posterior
commissure of Darkschewitsch whicli lies in the mesencephalon just rostral to the oculomotor
nucleus and which, according to Cajal, may or may not send fibers into the medial longitudinal
bundle.
154
THE NERVOUS SYSTEM
the platysma and muscles of the face. These fibers emerge from the dorsal
surface of the nucleus and run dorsomedially toward the floor of the fourth
ventricle. Somewhat widely separated at first, they become united on the
medial side of the abducens nerve into a compact strand, which as the genu of
the facial nerve partly encircles this nucleus, and which then runs ventrolateraUy
between the spinal tract of the trigeminal nerve and its own nucleus toward
its exit from the brain (Figs. 108, 124).
Anterior medullary velum
Medial longitudinal fasckulus
Ventral spinocerebellar tract
Trapezoid body:
Superior olive,
Lateral lemniscus
Brachium pontis
Fourth ventricle
Brachium conjunctiwim
Mesencephalic root of trigem-
inal nerve
Motor nucleus of trigeminal
nerve
Sensory nucleus of trigem-
inal nerve
Medial lemniscus
Superficial stratum of pons
Trigeminal nerve
Corticospinal and corlico-
pontine tracts
Nuclei pontis
Fig. 110. Section through the pons of a child at the level of the motor nucleus of the trigeminal
nerve. Pal-Weigert method. (X 4.)
The nucleus of the abducens nerve along with the genu of the facial pro-
duces a rounded elevation in the rhomboid fossa, known as the facial colliculus
(Figs. 89, 108). It is a spheric mass of gray matter containing the cells of origin
of the fibers which innervate the lateral rectus. These emerge from the dorsal
and medial surfaces of the nucleus and run ventrally more or less parallel to the
median raphe toward their exit at the lower border of the pons.
The Nuclei of the Trigeminal Nerve. In transverse section through approxi-
INTERNAL STRUCTURE OF THE PONS 155
mately the middle of the pons we encounter the fibers of the trigeminal nerve
and two associated masses of gray matter, the motor and main sensory nuclei
of that nerve (Fig. 110). These are located close together in the dorsolateral
part of the reticular formation near the groove between the middle and supe-
rior cerebellar peduncles. Of the two, the sensory nucleus is the more superficial.
It is, in reality, not a new structure, but rather the enlarged rostral extremity
of the column of gray matter which we have followed upward from the sub-
stantia gelatinosa Rolandi of the spinal cord and have designated as the nucleus
of the spinal tract of the trigeminal nerve (Figs. 98, 101). On its medial side is
found the motor nucleus, a large oval mass of gray matter from the cells of which
arise the motor fibers for the muscles of mastication. Some of the fibers of the
trigeminal nerve, passing between these two nuclei, are continued as the mesen-
cephalic root of the trigeminal nerve (Figs. 110, 111). Reaching the gray matter
in the lateral wall of the rostral part of the fourth ventricle, this bundle of fibers
turns rostrally along the medial side of the brachium conjunctivum (Fig. 112).
It extends into the mesencephalon in the lateral part of the gray matter which
surrounds the cerebral aqueduct (Fig. 114). The fibers of this root take origin
from unipolar cells scattered along its course and known as the mesencephalic
nucleus of the trigeminal nerve.
It will be apparent from this description that there are four nuclear masses
associated with the trigeminal nerve, namely, the nucleus of the spinal tract,
and the main sensory, motor, and mesencephalic nuclei. The relations which
each of these groups of cells bear to the fibers of the trigeminal nerve are illus-
trated in Fig. 111. Note that those fibers which arise from cells in the semi-
lunar ganglion divide into short ascending and long descending branches. The
former end in the main sensory nucleus; while the latter run in the spinal tract
of the trigeminal nerve and end in the nucleus which accompanies it.
The brachium conjunctivum or superior cerebellar peduncle (Fig. 89) is seen
in sections through the rostral half of the pons, where it enters into the lateral
boundary of the fourth ventricle. It is a large strand of fibers which runs from
the dentate nucleus of the cerebellum to the red nucleus of the mesencephalon
(Fig. 115). As it emerges from the white center of the cerebellum this brachium
is superficially placed, with its ventral border resting on the tegmental portion
of the pons (Fig. 110). To its dorsal border is attached a thin plate of white
matter, the anterior medullary velum, which roofs in the rostral part of the
fourth ventricle. As the brachium ascends toward the mesencephalon it sinks
deeper and deeper into the dorsal part of the pons until it is entirely submerged
156
THE NERVOUS SYSTEM
(Fig. 112). Near the rostral border of the pons it assumes a crescentic outline
and lies in the lateral part of the reticular formation. From its ventral border
Fig. 111. Diagram of the nuclei and central connections of the trigeminal nerve: A, Semi-
lunar ganglion; B, mesencephalic nucleus, N. V.; C, motor nucleus, N. V.; D, motor nucleus, N.
VII; E, motor nucleus, N. XII; F, nucleus of the spinal tract of N. V; G, sensory fibers of the sec-
ond order of the trigeminal path ; a, ascending and b, descending branches of the sensory fibers,
N. V; c, ophthalmic nerve; d, maxillary nerve; e, mandibular nerve. (Cajal.)
fibers stream across the median plane, decussating with similar fibers from the
opposite side. This is the most caudal portion of the decussation of the brachium
INTERNAL STRUCTURE OF THE PONS
conjunctivum, which increases in volume as it is followed rostrally, reaching its
maximum in the mesencephalon at the level of the inferior colliculi. In this
decussation the fibers of the brachium undergo a complete crossing.
The ventral spinocerebellar tract, which has made its way through the retic-
ular formation of the pons, turns dorsolaterally near the rostral end of the
pons, winds around the brachium conjunctivum, and enters the anterior medul-
lary velum, in which it descends to the vermis of the cerebellum (Figs. 110,
149).
Fourth ventricle
Dorsal longitudinal fasciculus
Medial longitudinal fasciculus
Thalamo-olwary
Brachium conjunctivum
tract
Decussation of brachium
conjunctivum
Trochlear nerve
Mesencephalic root of trigeminal
nerve
Lateral lemniscus and nucleus
Dorsal nucleus oflegmentum
Ventral nucleus of
tegmentum
Nucleus centralis superior
Medial lemniscus
Pons
Fig. 112. Dorsal half of a section through the rostral part of the human pons. The index
line to the mesencephalic root of the trigeminal nerve does not quite reach that structure. Pal-
Weigert method.
The lateral lemniscus is an important tract of fibers which we have already
traced from the cochlear nuclei by way of the trapezoid body and striae medul-
lares acusticae. It first takes definite shape about the middle of the pons, where
it is situated lateral to the medial lemniscus (Fig. 110). As it ascends it becomes
displaced dorsolaterally until it occupies a position on the lateral aspect of the
brachium conjunctivum (Fig. 112). In this position there is developed in con-
nection with it a collection of nerve-cells, the nucleus of the lateral lemniscus,
to which its fibers give off collaterals.
CHAPTER XI
Lamina quadrigemina
Cerebral aqueduct /
Central gray stratum-^., .(^ \J
Tegmentum
THE INTERNAL STRUCTURE OF THE MESENCEPHALON
A DIAGRAM of a transverse section through the rostral part of the mesen-
cephalon will make clear the relation of the various parts of the midbrain to
each other (Fig. 113). The cerebral aqueduct is surrounded by a thick lamina
of gray matter, the central gray stratum (stratum griseum centrale). Dorsal to
this lies the lamina quadrigemina, a plate of mingled gray and white matter
which bears four rounded elevations, the corpora quadrigemina. The ventral
part of the midbrain is formed by
the cerebral peduncles, each of which
is separated into two parts by a
lamina of pigmented gray substance,
known as the substantia nigra.
Dorsal to this the peduncle consists
of reticular formation continuous
with that of the pons and known as
the tegmentum. Ventral to the sub-
stantia nigra is a thick plate of longitu-
dinal fibers, called the basis pedunculi,
composed of fibers which are continuous with the longitudinal fasciculi of the
pons.
The Tegmentum. The dorsal portion of the pons is directly continuous
with the tegmentum of the mesencephalon. Both are composed of reticular
formation, consisting of interlacing longitudinal and transverse fibers grouped
in fine bundles and separated by minute masses of gray substance, in which are
embedded important nuclei and fiber tracts. In the caudal part of the mid-
brain and the rostral part of the pons are four cellular masses the locations of
which are indicated in Fig. 112. They are the dorsal nucleus of the raphe, the
superior central nucleus, the ventral tegmental nucleus, and the dorsal tegmental
nucleus. The latter is a collection of small cells in the central gray substance, sep-
arated from the ventral tegmental nucleus by the medial longitudinal bundle.
Both the ventral and dorsal tegmental nuclei receive fibers from the mammillary
body (tractus mamillotegmentalis) , and within the dorsal one there also ter-
Basis pedunculi'"
Substantia nigra--''
Fig. 113. Diagrammatic cross-section through
the human mesencephalon.
THE INTERNAL STRUCTURE OF THE MESENCEPHALON
159
minate fibers from the interpeduncular ganglion (Fig. 211). The tegmentum
contains many longitudinal fiber tracts which are continued into it from the dor-
sal part of the pons. The most conspicuous of these is the brachium conjunc-
tivum.
The Decussation of the Brachia Conjunctiva. In the sections of the pons we
saw that, as the brachia conjunctiva ascend toward the mesencephalon, they
sink deeper and deeper into the pars dorsalis pontis (Fig. 112). When they
reach the level of the inferior colliculi of the corpora quadrigemina they are
Aqueduct of cerebrum
Mesencephalic root of N. V
Medial longitudinal
fasciculus
Decussation of brachium
conjunctivum
Interpeduncular fossa
Szibstantia nigra
Commissure of inferior colliculi
Inferior quadrigeminal brachium
Nucleus of inferior colliculus
Lateral lemniscus
Trochlear nerve
Thalamo-olivary tract
Nucleus of trochlear nerve
Medial lemniscus
Basis pedunculi
Posterior perforated substance
Pons
Fig. 114. Section through the mesencephalon of a child at the level of the inferior colliculus.
Pal-Weigert method. (X 4.)
deeply placed in the tegmentum; and here they cross the median plane in the
decussation of the brachium conjunctivum (Fig. 114). After crossing, each brach-
ium turns rostrally and forms a rounded bundle of ascending fibers, which al-
most at once comes into relation with the red nucleus (Fig. 116). Many of the
fibers enter this nucleus directly, while others are prolonged over its surface to
form a capsule that is best developed on its medial surface. While the majority
of these fibers ultimately end in the red nucleus, some reach and end within the
ventral part of the thalamus (Fig. 115). By way of summary we may repeat
that the fibers of the brachium conjunctivum, or at least the greater part of them,
i6o
THE NERVOUS SYSTEM
arise in the dentate nucleus of the cerebellum; they cross the median plane
in the tegmentum at the level of the inferior colliculi and end either in the red
nucleus or in the thalamus.
According to Cajal (1911) the fibers of the brachium conjunctivum give off two sets of
descending branches, which he has seen in Golgi preparations of the mouse, rabbit, and cat.
The first group are collaterals given off as the brachium enters the dorsal part of the pons
and before its decussation (Fig. 115). They descend into the pons and medulla oblongata
and constitute a direct descending tract from the dentate nucleus of the cerebellum to the
reticular formation of the pons and medulla oblongata. The second group of descending
Rubrospinal tract x
Rubroreticular tract v
From frontal lobe and corpus strialum
Thalamus
Red nucleus
Brachium conjunctivum
Dentate nucleus
\
Pons
Rubrospinal tract
Medulla oblongata
Reticulospinal tract
Spinal cord
Fig. 115. Diagram showing the connections of the red nucleus: A, Ventral tegmental
decussation; B, decussation of the brachium conjunctivum; Cand D, descending fibers from bra-
chium conjunctivum, before and after its decussation respectively.
branches is formed by the bifurcation of the fibers of the brachium conjunctivum just beyond
the decussation, and constitute a crossed descending tract from the dentate nucleus, which
can be followed by degeneration methods through the reticular formation of the brain stem
and probably into the anterior and lateral funiculi of the spinal cord (Fig. 115).
The red nucleus (nucleus ruber) is a very large oval mass of gray matter,
which in the fresh brain has a pink color. It is located on the path of the brach-
ium conjunctivum in the rostral part of the tegmentum (Fig. 116). In trans-
verse sections it presents a circular outline and can be followed from the level
of the inferior border of the superior colliculus into the hypothalamus. In its
caudal portion it contains great numbers of fibers derived from the brachium
THE INTERNAL STRUCTURE OF THE MESENCEPHALON l6l
conjunctivum, and stains deeply in Weigert preparations, but farther rostrally
these fibers are less numerous and the nucleus takes on more and more the ap-
pearance of gray substance.
Afferent fibers reach the red nucleus chiefly through the brachium con-
junctivum, but it also receives fibers from the cerebral cortex of the frontal
lobe and others from the corpus striatum (Fig. 115). These descending fibers
help to form the capsule of the nucleus and are most abundant along its medial
surface.
Efferent Fibers. From the cells of the red nucleus arise the fibers of the
rubrospinal tract, which after crossing the median plane descend into the spinal
cord. Other cells give origin to fibers, which decussate along with those of the
rubrospinal tract and terminate in the nuclei of the reticular formation and in
the nucleus of the lateral lemniscus. These form the tractus rubroreticularis
(Fig. 115). Other fibers from the red nucleus reach the thalamus.
The nerve-cells which are found in the red nucleus vary greatly in size. The smaller
ones have the character of the cells of the reticular formation and send their axons into the
tegmentum of the same and the opposite side. Another group of very large cells furnishes
the axons that constitute the rubrospinal tract. This collection of large cells is phylogenetic-
ally the older and forms the chief part of the red nucleus in the lower mammals. But in
man, where the two parts are rather sharply differentiated, the chief mass is composed of
the smaller cells.
The red nucleus may be regarded as an especially highly developed portion of the motor
nuclei of the tegmentum. In the lower mammals it serves as a center through which the
cerebellum can influence the motor functions of the spinal cord and medulla oblongata.
In man it has the same function, but is also more closely linked with the reticular formation
of the pons by way of the rubroreticular tract. It is a significant fact that in man where
the rubrospinal tract is relatively small the rubroreticular tract is especially well developed.
This suggests the possibility that impulses from the red nucleus may be relayed through the
reticular nuclei of the pons to the spinal cord (Fig. 115).
The Tegmental Decussations. At the level of the superior colliculus and
between the two red nuclei the median raphe presents an unusual number of
crossing fibers (Fig. 116). Among these are included the dorsal tegmental de-
cussation (fountain decussation of Meynert) and the ventral tegmental decussa-
tion (fountain decussation of Forel). The latter is composed of fibers from the
red nucleus, which, after crossing the median plane, descend through the brain
stem into the lateral funiculus of the spinal cord as the rubrospinal tract (Fig.
115). The dorsal tegmental decussation is composed of fibers which arise in the
superior colliculi of the corpora quadrigemina, sweep in broad curves around the
central gray stratum, and after crossing the median plane in the dorsal part of
the raphe, go to form the tectobulbar and tectospinal tracts.
162
THE NERVOUS SYSTEM
The median longitudinal fasciculus is more conspicuous in the mesencephalon
than in other parts of the brain stem, but it occupies the same relative position,
that is, near the median plane close to the central gray matter. At the level of
the superior colliculus it forms a rather broad obliquely placed lamina, extending
dorsolaterally from the median raphe, which together with the corresponding
lamina of the opposite side produces in transverse sections a V-shaped figure
(Fig. 116). The apex of this V is directed ventrally; and included between its
two limbs are the oculomotor nuclei. At the level of the inferior colliculi the
Stratum zonale
Stratum griseum
Stratum opticum
Stratum Iemnisci'~r7 l
Stratum profundum-^
Aqueduct of cere
brum
Medial lemnis-
Superior colliculus
Nucleus of oculomotor nerve
Medial longitudinal fasciculus
Thalamo-olivary tract
Inf. quadrigeminal brack.
Med. gen. body
Dorsal tegmental decussation
Ventral tegmental decussation
Red nucleus
Oculomotor nerve
gf Basis pedunciili
Substantia nigra
Fig. 116. Section through the mesencephalon of a child at the level of the superior colliculus.
Pal-Weigert method. (X 4.)
medial longitudinal fasciculus lies immediately ventral to the nucleus of the
trochlear nerve (Fig. 114). In the pons the nucleus of the abducens nerve is
placed on its dorsolateral border. The close relation of this fascicle to the nuclei
for the motor nerves of the eye is of considerable significance, since according
to the law of neurobiotaxis (p. 179) it is an expression of the fact that the majority
of the afferent fibers to these nuclei come from this fascicle. This bundle of
fibers, the composition of which is discussed on pages 152 and 329, is a chief
factor in the reflex control of the movements of the eyes, and especially in the
coordination of these movements with those of the head and neck.
THE INTERNAL STRUCTURE OF THE MESENCEPHALON 163
The Lemnisci. In sections through the rostral border of the pons the two
lemnisci form a broad curved band in the ventral and lateral portions of the
tegmentum. The fibers of the lateral lemniscus are cut obliquely, indicating
that they have begun to turn dorsally toward the inferior colliculus (Fig. 112).
On entering the midbrain this lateral portion of the fillet separates from the
medial lemniscus and runs toward the corpora quadrigemina, where it forms a
capsule for the nucleus of the inferior colliculus (Fig. 114). Some of these
fibers are prolonged beyond the nucleus and decussate with similar fibers from
the opposite side. A large proportion of the fibers of the lateral lemniscus end
in the inferior colliculus, but others form the inferior quadrigeminal brachium
(Fig. 114), through which they reach the medial geniculate body (Figs. 116, 134).
In the mesencephalon the lateral lemniscus, which, it will be remembered, is the
central auditory tract from the cochear nuclei, is joined by the fibers of the
spinotectal tract; and these run with it to the corpora quadrigemina.
The medial lemniscus, or bulbothalamic tract from the gracile and cuneate
nuclei of the opposite side, is continued through the tegmentum of the mesen-
cephalon to end in the lateral nucleus of the thalamus (Fig. 235) . Incorporated
with it in this upper part of its course are the fibers of the spinothalamic tract
and a portion of the central sensory tract of the trigeminal nerve (Figs. 132, 234).
In the caudal part of the mesencephalon this broad band of longitudinal fibers
occupies the ventrolateral portion of the tegmentum (Fig. 114); but at the level
of the superior colliculus it has been displaced dorsolaterally by the red nucleus.
Here it lies not far from the medial geniculate body and inferior quadrigeminal
brachium (Fig. 116).
The Central Gray Stratum. The cerebral aqueduct is lined by ependymal
epithelium and surrounded by a thick layer of gray matter, the central gray
stratum, which, because of its paucity in myelinated fibers, is nearly colorless in
Weigert preparations. This layer is continuous with the gray matter surround-
ing the third ventricle, on the one hand, and with that covering the rhomboid
fossa on the other. Numerous nerve-cells of various size and shape are scat-
tered through this central gray substance; and, in addition, there are three
compact groups of cells, which are the nuclei of the oculomotor and trochlear
nerves and of the mesencephalic root of the trigeminus.
The nucleus of the trochlear nerve contains the cells of origin of the motor
fibers for the superior oblique muscle of the eye. It is a small oval mass situated
in the ventral part of the central gray stratum at the level of the inferiof collic-
ulus (Fig. 1 14) . The fibers of the trochlear nerve emerge from the dorsolateral
164 THE NERVOUS SYSTEM
aspect of this nucleus, curve dorsally around the central gray matter, and decus-
sate in the anterior medullary velum (Fig. 112).
The nucleus of the oculomotor nerve is composed of the cells of origin of
the motor fibers for all of the ocular muscles except the superior oblique and
lateral rectus. It lies in the ventral part of the central gray substance beneath
the superior colliculus (Fig. 116). This nucleus, a part of which occupies a
median position and supplies fibers to the nerves of both sides, is 6 or 7 mm.
long and extends from a little beyond the rostral limit of the mesencephalon to
the nucleus of the trochlear nerve, from which it is not sharply separated. From
the nucleus the fibers of the oculomotor nerve stream forward through the
tegmentum and red nucleus. They emerge through the oculomotor sulcus along
the ventromedial surface of the basis pedunculi.
The interpeduncular ganglion is a median collection of nerve-cells in the
posterior perforated substance situated between the two cerebral peduncles near
the border of the pons (Fig. 1 14) . It receives fibers from the habenular nucleus
of the epithalamus by way of the fasciculus retroflexus of Meynert; and from
it spring fibers that run to the dorsal nucleus of the tegmentum (Fig. 211).
The substantia nigra is a broad thick plate of pigmented gray matter, which
separates the basis pedunculi from the tegmentum and extends from the border
of the pons throughout the length of the mesencephalon into the hypothalamus.
In transverse section it presents a semilunar outline. Its medial border is super-
ficial in the oculomotor sulcus and is thicker than the lateral border, which
reaches the lateral sulcus of the mesencephalon. Its constituent nerve-cells,
irregular in shape and deeply pigmented, send their axons into the tegmentum.
But we are still ignorant as to the destination these may have; and the func-
tion of the substantia nigra is equally obscure. It receives collaterals from the
corticifugal fibers of the basis pedunculi. Furthermore, there terminates within
it a bundle, consisting of both direct and crossed fibers from the corpus striatum,
the strionigral tract (Fig. 117).
The basis pedunculi is a broad compact strand, crescentic in transverse sec-
tion, which consists of longitudinal fibers of cortical origin. These are con-
tinued from the internal capsule into the longitudinal bundles of the pons
through the basis pedunculi. It consists of four tracts. The medial and lat-
eral fifths are occupied by fibers which terminate in the nuclei pontis. Those
of the medial one-fifth arise from the cortex of the frontal lobe of the cerebral
hemisphere and constitute the frontopontine tract. Other fibers, arising from
the temporal lobe, form the temporopontine tract and occupy the lateral one-
THE INTERNAL STRUCTURE OF THE MESENCEPHALON
fifth of the basis pedunculi. The intermediate portion, approximately three-
fifths, is formed by the corticospinal tract, the fibers of which after giving off
collaterals to the nuclei pontis are continued into the pyramids of the medulla
oblongata and thence into the spinal cord. Many of the fibers of the cortico-
bulbar tract are intermingled with the more medially placed corticospinal fibers;
but even at this level two large fascicles destined for the nuclei of the cranial
nerves have separated from the main strand of motor fibers (Dejerine, 1914).
These have been called the medial and lateral corticobulbar tracts (Figs. 106,
117).
The Corpora Quadrigemina. The rostral portion of the midbrain roof or
tectum mesencephali is in all vertebrates an end-station for the optic tracts. In
the lower vertebrates there are but two elevations in the roof, the optic lobes or
corpora bigemina, and these, which correspond in a general way to the superior
Temporopontine tract
Tr. corticobulbaris lot.
Strionigral tract
Corticospinal tract
Frontopontine tract Tr. corlicobulbaris med.
Fig. 117. Diagram of the basis pedunculi.
colliculi, are visual centers (Fig. 13). In mammals the development of a spir-
ally wound cochlea is associated with the appearance of two additional eleva-
tions, the inferior colliculi, within which many of the fibers of the central audi-
tory path terminate. The entire tectum receives fibers from the spinal cord
and medulla oblongata and sends other fibers back to them ; it also receives fibers
from the cerebral cortex. It contains important reflex centers, those in the
superior colliculus being dominated by visual, those in the inferior colliculus
by auditory, impulses.
The inferior colliculi or inferior quadrigeminal bodies each contain, in addi-
tion to the laminated gray matter of the tectum, a large gray mass, oval in
transverse section, and known as the nucleus of the inferior colliculus (Fig. 114).
1 66
THE NERVOUS SYSTEM
The lateral lemniscus has been traced to this nucleus, and while some of the
fibers plunge directly into it, others sweep around it to form a capsule, within
which it is enclosed. The majority of these fibers ultimately end in this nu-
cleus, but some pass beyond it, reach the median plane, and decussate with sim-
ilar fibers from the opposite side (Fig. 118). The ramifications of fibers from the
lateral lemniscus form an intricate interlacement within the nucleus, and
throughout this network are scattered many nerve-cells of various shapes and
Fig. 118. Semidiagrammatic section through the inferior colliculus of the mouse: A, Nucleus
of inferior colliculus; B, gray matter of the lamina quadrigemina; C, inferior quadrigeminal bra-
chium; D, central gray substance; K, decussation of the brachium conjunctivum; a, b, c, d, fibers
of the lateral lemnisus. Golgi method. (Cajal.)
sizes. On the medial side of this circumscribed nuclear mass we find some of
the laminated gray matter of the tectum, within which are embedded large mul-
tipolar cells with axons directed ventrally in the stratum profundum. These
partially encircle the central gray matter and after undergoing a partial decus-
sation enter the tectobulbar and tectospinal tracts.
The inferior quadrigeminal brachium begins on the lateral side of the nucleus
of the inferior colliculus and consists of fibers from the lateral lemniscus which
THE INTERNAL STRUCTURE OF THE MESENCEPHALON 167
run to and terminate within the medial geniculate body (Figs. 114, 116). The
fibers of the lateral lemniscus carry auditory impulses from the terminal nuclei
of the cochlear nerve. Some of these terminate in the inferior colliculus and
are concerned with reflexes in response to sound. Other fibers, some of which
are branches of those to the inferior colliculus, run to the medial geniculate
body, from which the impulses that they carry are relayed to the cerebral cor-
tex. The inferior quadrigeminal brachium also contains fibers of cortical origin,
chiefly from the temporal lobe, which end within the inferior colliculus (Beevor
and Horsley, 1902).
The superior colliculi, or superior quadrigeminal bodies, are composed of
laminated gray matter. Each consists of four superimposed, dor sally convex
layers (Fig. 116). The most superficial of these is a thin lamina with many
transversely coursing nerve-fibers, the stratum zonale. The second layer is much
thicker, contains few myelinated fibers, and is known as the stratum griseum.
The third and fourth layers, stratum opticum and stratum lemnisci, are rich in
myelinated fibers. The majority of the afferent fibers of the superior colliculus
come from the optic tract by way of the superior quadrigeminal brachium and
enter the stratum opticum. Many of these end in the superimposed stratum
griseum. The superior colliculus also receives fibers from the cerebral cortex
and from the spinotectal tract.
It has been generally supposed that the fibers of the stratum zonale come from the
optic tract, but according to Cajal (1911) this cannot be the case, since they remain intact
in animals which have been operated on in such a way as to produce degeneration of the optic
fibers. According to him it is also probable that the fibers from the cerebral cortex, which
reach the colliculus by way of the superior quadrigeminal brachium, end in the stratum
lemnisci. The fibers of the spinotectal tract run with the lateral lemniscus in the upper part
of its course and enter the superior colliculus by way of the stratum profundum.
The tectobulbar and tectospinal tracts have their origin within the tectum of
the mesencephalon, more of the fibers coming from the superior than from the
inferior colliculi. These fibers, arising from cells in more superficial layers, are
assembled in the stratum profundum and sweep ventrally in broad curves around
the central gray substance (Figs. 116, 118). The majority of the fibers, after
crossing the median plane in the dorsal tegmental decussation, run in a caudal
direction just ventral to the medial longitudinal bundle in the tectospinal tract.
They give off collaterals to the reticular formation and the red nucleus. But
some of them, instead of taking part in this decussation, leave the mesencephalon
by way of the lateral lemniscus of the same side, constituting the lateral tecto-
bulbar and tectospinal tracts (Cajal, 1911; Edinger, 1911).
CHAPTER XII
THE CRANIAL NERVES AND THEIR NUCLEI
THE cranial nerves contain, in addition to the general somatic and visceral
components, which were encountered in the study of the spinal nerves, also
other functional groups of fibers of more restricted distribution and specialized
function. These special somatic and visceral components supply the organs of
special sense and the visceral musculature, derived from the branchial arches,
which differs from other visceral musculature in that it is striated. The fibers
which supply this special musculature are designated as special visceral efferent
fibers. The eye and ear, being special somatic sense organs, are supplied by
special somatic afferent fibers. The olfactory mucous membrane and the taste
buds are special visceral sense organs and are supplied by special visceral af-
ferent fibers.
From what has been said it will be evident that there are seven distinct
functional components in the cranial nerves, namely: somatic efferent, general
somatic afferent, special somatic afferent, general visceral efferent, special vis-
ceral efferent, general visceral afferent, and special visceral afferent components
(Figs. 119, 120). No single nerve contains all seven types of fibers and the
individual cranial nerves vary greatly in their functional composition. On
entering the brain a nerve breaks up into its several components, which separate
from each other and pass to their respective nuclei, enumerated below. These
nuclei may be widely separated in the brain stem. Fibers having the same func-
tion tend to be associated together within the brain irrespective of the nerves
to which they belong. For example, all the visceral afferent fibers of the facial,
glossopharyngeal, and vagus nerves are grouped in the tractus solitarius (Fig.
120, yellow). The nerve-cells, with which the fibers of the several functional
varieties are associated within the brain stem, are arranged in longitudinal
nuclear columns. The analysis of the cranial nerves into their functional com-
ponents has involved a great amount of labor which has been carried through
for the most part by American investigators. Among those who have made
important contributions to this subject may be mentioned the following: Gas-
kell (1886), Strong (1895), Herrick (1899), Johnston (1901), Coghill (1902),
Norris (1908), and Willard (1915).
1 68
Special somatic afferent _
nucleus
General somatic afferent
nucleus
Alar lamina'
Visceral afferent nucleus-
General visceral efferent__
nucleus
Special visceral efferent
nucleus
Basal lamina"
Somatic efferent nucleus *~
Somatic muscle
Sympathetic ganglion
Visceral mucous membrane
Smooth muscle
XN Sensory ganglion
Branchial muscle
Fig. 119.
Sensor v nucleus N. V*
V
Nucleus of abducens nerve\ \
Facial nerve, \
Vestibular nuclei \ \
\ '
Vestibular ganglion
and nerve
DC
Bulbar rootlet of accessory nerve
Spinal root of accessory nerve''
Nucleus ambiguus-'
Tractus solitarius
Nucleus of Edinger-Westphal
Nucleus of oculomotor nerve
Nucleus of trochlear nerve
M esencephalic nucleus N. V
Trigeminal nerve
and semilunar
ganglion
r . Spinal tract and
nucleus N. V
. Cochlear nuclei
Spiral ganglion
and cochlear
.. Glossopharyn-
geal nerre
Vagus nerve
. salivatorius superior
'. salivatorius inferior
Dorsal motor nucleus N. X
Cervical spinal nerve
Fig. 120.
Figs. 119 and 120. Diagrams showing the origin, course, and termination of the functional
components of the cranial nerves. Somatic afferent and efferent, red; visceral afferent, yellow;
general visceral efferent, black; special visceral efferent, blue. Fig. 119 shows the locations of the
several functional cell columns in a section through the medulla oblongata of a human embryo and
the peripheral terminations of the several varieties of fibers. Fig. 120, dorsal view of the human
brain stem, showing the location of the nuclei and the intramedullary course of the fibers of the
cranial nerves. 169
THE NERVOUS SYSTEM
Longitudinal Nuclear Columns. In a previous chapter we learned that at
an early stage in its development the lateral wall of the neural tube consists of a
dorsal or alar and a ventral or basal plate, separated by a groove, the sulcus
limitans (Fig. 119). The sensory nuclei of the cranial nerves develop within the
alar plate and the motor nuclei within the basal plate. In the rhombencephalon
both plates come to lie in the floor of the fourth ventricle, the alar occupying
the more lateral position. And, in spite of the changes of position which occur
during development, the sensory nuclei retain, on the whole, a lateral, and the
motor nuclei a more medial, location. From the basal plate there differentiate
a somatic and a visceral column of efferent nuclei, and from the alar plate a
visceral and a somatic column of afferent nuclei.
The somatic e/erent column includes the nuclei of those motor nerves which
supply the striated musculature derived from the myo tomes, i.e., the extrinsic
muscles of the eye and the musculature of the tongue (Figs. 119-121).
The visceral efferent column undergoes subdivision into: (1) a ventrolateral
column of nuclei, from which arise the special visceral efferent fibers to the striated
visceral or branchial musculature, and which includes the nucleus ambiguus and
the motor nuclei of the fifth and seventh nerves; and (2) a more dorsally placed
group for the innervation of involuntary musculature and glandular tissue, of
which the dorsal motor nucleus of the vagus is the chief example. The former
may be called the special visceral efferent and the latter the general visceral ef-
ferent column.
The visceral afferent column is represented by the nucleus of the tractus
solitarius, within which end the afferent fibers from the visceral mucous membrane
and the taste buds, i. e., both the general and special visceral afferent fibers.
The somatic afferent column splits into two: a general somatic afferent column,
within which terminate the sensory fibers from the skin; and a special somatic
group of nuclei for the reception of the fibers of the acoustic nerve and, in aquatic
vertebrates, of the lateral line nerves also.
THE SOMATIC EFFERENT COLUMN
As can be seen by reference to Figs. 101, 108, 114, and 116 the nuclei of the
hypoglossal, abducens, trochlear, and oculomotor nerves are arranged in linear
order in the central gray matter near the median plane. They represent the
continuation into the medulla oblongata of the large cells of the anterior column
of the spinal cord. The cells of these nuclei are large and multipolar with
well-developed Nissl bodies (Fig. 126). From them arise large myelinated
THE CRANIAL NERVES AND THEIR NUCLEI 171
fibers, which innervate the striated musculature derived from the myotomes.
This group of nuclei is indicated in red in Fig. 120 and by small circles in Figs.
121 and 122.
The nucleus of the oculomotor nerve is an elongated mass of cells in the cen-
tral gray matter ventral to the cerebral aqueduct at the level of the superior
colliculus (Figs. 121, 122). Even a superficial examination shows that it is
divided into a lateral paired and a medial unpaired portion (Fig. 116). The
Nuc. Ill E-W.
Nuc. Ill lat.
Nuc. Ill med.
Nuc. IV
Velum medul-
lare superiua
Nuc. mot. V
Nuc. VI
Nuc. mot. VII
Nuc. sal. sup.
Nuc. sal. inf.
Ala cinerea
Nuc. dorsal. X
Nuc. ambiguua
Nuc. XII
Colliculus sup.
Corp. genicula-
tum mediale
Nuc. com.
Cajal
Fig. 121. Dorsal view of the human brain stem with the positions of the cranial nerve nuclei
projected upon the surface. Sensory nuclei on the right side, motor nuclei on the left. Circles
indicate somatic efferent nuclei; small dots, general visceral efferent nuclei; large dots, special
visceral efferent nuclei; horizontal lines, general somatic sensory nuclei; cross-hatching, visceral
sensory nuclei; stipple, special somatic sensory nuclei. (Herrick.)
lateral groups of cells spreads out upon the surface of the medial longitudinal
bundle, extends throughout the entire length of the nucleus, and may be divided
into ventral and dorsal portions (Fig. 123). The medial group of cells is placed
exactly in the median plane and is found only in the rostral half of the nucleus.
Dorsolateral from this median group, and restricted to the most rostral part of
the nucleus, is a collection of small cells which form the nucleus of Edinger-
Westphal. This is a visceromotor nucleus and will be considered elsewhere.
172
THE NERVOUS SYSTEM
The fibers from the medial nucleus enter both right and left nerves. Some
from the caudal portion of the dorsal division of the lateral nucleus cross the
median plane. The others remain uncrossed. After sweeping in broad curves
through the tegmentum and red nucleus the fibers emerge through the oculo-
motor sulcus. All of the extrinsic muscles of the eye except the lateral rectus
and superior oblique are supplied by the medial and lateral groups of cells just
described.
t , Nucleus of Edinger-Westphal
,. Nucleus of oculomotor nerve
--Corpora quadrigemina
Cerebral aqueduct
-Nucleus of trochlear nerve
Trochlear nerve
'Anterior medullary velum
'Motor nucleus N. V
. - 'Nticleus of facial nerve
, - Fourth ventricle
Nucleus of abducens nerve
Mesencephalon
Oculomotor nerve
Pans
Portia minor N. V-
Facial nerve
Abducens nerve- - '
Medulla oblongata-
- Nuc. salivatorius superior
- Nuc. salivatorius inferior
Nucleus of hypoglossal nerve
- - Dorsal motor nucleus N. X
.--Central canal
Nucleus ambiguus
Fig. 122. Motor nuclei of the cranial nerves projected on a median sagittal section of the
human brain stem. Circles indicate somatic efferent nuclei; small dots, general visceral efferent
nuclei; large dots, special visceral efferent nuclei.
As one might expect from the fact that the oculomotor nerve supplies several distinct
muscles, its nucleus seems to be made up of a number of more or less distinct groups of cells;
but the efforts to locate subordinate nuclei have given rise to contradictory results. The
most significant work in this field has been done by Bernheimer (1904), who extirpated in-
dividual eye muscles in monkeys and studied the resultant changes in the cells of the oculo-
motor nuclei. According to him, the various muscles are supplied by the lateral nucleus in
the following order, beginning at the rostral end: levator palpebrae superioris, rectus supe-
rior, rectus medialis, obliquus inferior, and rectus inferior. Bernheimer says that the fibers
for the rectus inferior are entirely crossed, those for the obliquus inferior are in greater part
crossed, those for the rectus medialis for the most part uncrossed, those for the rectus superior
and levator palpebrse superioris entirely uncrossed.
THE CRANIAL NERVES AND THEIR NUCLEI
173
V
The nucleus of the trochlear nerve has already been located in the central
gray matter ventral to the cerebral aqueduct at the level of the inferior collic-
ulus, close to the caudal extremity of the oculomotor nucleus (Figs. 114, 121,
122). The fibers of the trochlear nerve emerge from the dorsal and lateral
aspects of this nucleus, and, encircling the central gray matter along an angular
course which carries them also caudally, enter the anterior medullary velum,
decussate within it, and make their exit from its dorsal surface (Fig. 112). They
supply the superior oblique muscle.
The nucleus of the abducens nerve was encountered in the dorsal portion
of the pons as a spheric gray mass, which with the genu of the facial nerve forms
the facial colliculus of the rhomboid fossa (Figs.
108, 121, 122). The fibers of the abducens nerve
leave the nucleus chiefly on its dorsal and medial
surfaces and become assembled into several root
bundles, which are directed ventrally toward their
exit from the lower border of the pons near the
pyramid of the medulla oblongata. It supplies
the lateral rectus muscle.
The axons, which ramify within the three nuclei
for the motor nerves of the eye, are derived from
many sources. The most important of these
sources are the corticobulbar tract, the medial lon-
gitudinal bundle, and the tectospinal tract. The
nucleus of the abducens receives fibers also from
the central auditory apparatus through the pe-
duncle of the superior olive. These various fibers
provide for voluntary movements of the eyes, and
for reflex ocular movements in response to vestibular, visual, and auditory
impulses. The nuclei probably also receive branches from the central sensory
path of the fifth nerve.
The nucleus of the hypoglossal nerve is a slender cylindric mass of gray
matter nearly 2 cm. in length, extending from the level of the fovea inferior to
that of the decussation of the pyramids. We have already identified it in both
the open and the closed portions of the medulla oblongata (Figs. 99, 103). In the
floor of the fourth ventricle it lies beneath the trigonum hypoglossi, while more
caudally it lies ventral to the central canal (Figs. 121, 122). The root fibers
Fig. 123. Diagram of the
nuclei of the oculomotor nerve:
M, Median nucleus; E.W., nu-
cleus of Edinger-Westphal; V.L.,
D.L., ventral and dorsal portions
of the lateral nucleus. (Ober-
steiner.)
174 THE NERVOUS SYSTEM
are assembled into bundles which run ventrally toward their exit along the
lateral border of the pyramid.
A conspicuous plexus of myelinated fibers gives the hypoglossal nucleus a
characteristic appearance in Weigert preparations. Fibers from many sources
reach the nucleus and ramify within it. These include some from the cortico-
bulbar tract and others from the sensory nuclei of the fifth nerve and from the
nucleus of the tractus solitarius. The part which such fibers may play in reflex
movements of the tongue is illustrated in Fig. 92.
THE SPECIAL VISCERAL EFFERENT COLUMN
The special visceral efferent column of nuclei contains the cells of origin of
the motor fibers for the striated musculature derived from the branchial arches,
as distinguished from the general skeletal musculature that develops from
the myotomes. The branchial musculature includes the following groups of
muscles: the muscles of mastication, derived from the mesoderm of the first
branchial arch and innervated by the trigeminal nerve; the muscles of expression,
derived from the second or hyoid arch and innervated by the facial nerve; the
musculature of the pharnyx and larynx, derived from the third and fourth arches
and innervated by the glossopharyngeal, vagus, and accessory nerves; and prob-
ably also the sternocleidomastoid and trapezius muscles, innervated through the
spinal root of the accessory nerve. Some authors prefer to call this column,
which includes the motor nuclei of the fifth and seventh nerves and the nucleus
ambiguus, the lateral somatic column, because the cells in these nuclei and the
fibers which arise from them possess the characteristics of somatic motor cells
and fibers (Malone, 1913). The nuclei are composed of large multipolar cells
with well-developed Nissl bodies. These cells give origin to large myelinated
fibers which run through the corresponding nerve and terminate in neuromus-
cular endings in one or another of the muscles indicated above.
The motor nuclei of the fifth and seventh nerves and the nucleus ambiguus
of the ninth, tenth, and eleventh nerves form a broken column of gray matter,
located in the ventrolateral part of the reticular formation of the pons and
medulla oblongata some distance beneath the floor of the fourth ventricle (Figs.
121, 122). The cells of this column and the special visceral efferent fibers which
arise from them have been colored blue in Figs. 119 and 120.
The motor nucleus of the trigeminal nerve lies on the medial side of the
main sensory nucleus of that nerve, and is located at the level of the middle
of the pons in the lateral part of the reticular formation some distance from the
THE CRANIAL NERVES AND THEIR NUCLEI
175
ventricular floor (Figs. 110, 121, 122). The fibers, which take their origin here,
are collected in the motor root or portio minor of the fifth nerve and run with its
mandibular division to the muscles of mastication. Within the nucleus there
terminate fibers from the corticobulbar tract and many fibers, chiefly collaterals,
from the central sensory tract of the trigeminal nerve. It also receives collat-
erals from the mesencephalic root of the trigeminal and from other sources
(Fig. 131).
The motor nucleus of the facial nerve is located in the ventrolateral part
of the reticular formation of the pons near its caudal border (Figs. 108, 121,
122). Its constituent cells are arranged so as to form a varying number of sub-
groups which may possibly be concerned with the innervation of individual facial
muscles.
Root of facial nerve, first part
Abducens nucleus
Root of facial nerve, genu
Root of facial nerve, second part
Facial nucleus
Nucleus of abducens nerve
Root filaments of abducens nerve
Stalk of superior olive
Root of facial nerve, first part
Spinal root and nucleus N. V
Nucleus of facial nerve
Root of facial n., sec.
Superior olive [part
Abducens nerve
Fig. 124. Diagram of the root of the facial nerve, shown as if exposed by dissection in a thick
section of the pons.
From the dorsal aspect of this nucleus there emerge a large number of fine
bundles of fibers, directed dorsomedially through the reticular formation. These
rather widely separated bundles constitute the first part of the root of the facial
nerve (Fig. 124). Beneath the floor of the fourth ventricle the fibers turn sharply
rostrad and are assembled into a compact strand of longitudinal fibers, often
called the ascending part of the facial nerve. This ascends along the medial side
of the abducens nucleus dorsal to the medial longitudinal bundle for a consid-
erable distance (5 mm.). The nerve then turns sharply lateralward over the
dorsal surface of the nucleus of the abducens nerve, and helps to form the eleva-
tion in the rhomboid fossa, known as the facial colliculus. This bend around
the abducens nucleus, including the ascending part of the facial nerve, is known
176
THE NERVOUS SYSTEM
as the genu. The second part of the root of the facial nerve is directed ventro-
laterally and at the same time somewhat caudally, passing close to the lateral
side of its own nucleus, to make its exit from the lateral part of the caudal
border of the pons (Fig. 108).
Fibers from many sources terminate in the facial nucleus in synaptic rela-
tion with its constituent cells. Those from the corticobulbar tract place the
facial muscles under voluntary control. Others are collaterals from the sec-
ondary sensory paths in the reticular formation and are concerned with bulbar
reflexes. Some of these collaterals are given off by fibers arising in the trapezoid
body and carry auditory impulses. Others are collaterals of fibers arising in
the nucleus of the spinal tract of the fifth nerve; and still others are given off by
ascending sensory fibers from the spinal cord (Cajal, 1909).
Bulbar rootlets of accessory nerve ^-^
Foramen magnum-\.
Spinal root of accessory nerve
-. Vagus nerve
-'Jugular foramen
Internal ramus \ *
77 , ; } Accessory nerve
External ramus / *
1
Fig. 125. Diagram of the roots of the vagus and accessory nerves.
The nucleus ambiguus is a long slender column of nerve-cells, extending
through the greater part of the length of the medulla oblongata in the ventro-
lateral part of the reticular formation (Figs. 103, 121, 122). Its constituent
cells give rise to the special visceral efferent fibers that run through the glosso-
pharyngeal, vagus, and accessory nerves to supply the musculature of the
pharynx and larynx. It reaches from the border of the pons to the motor de-
cussation, but is most evident in transverse sections through the caudal part of
the rhomboid fossa. Here it can be found in the reticular formation ventral to
the nucleus of the spinal root of the trigeminal nerve. The fibers arising from
its cells are at first directed dorsally; then curving laterally and ventrally they
join the root bundles of the ninth, tenth, and eleventh nerves with which they
THE CRANIAL NERVES AND THEIR NUCLEI 177
emerge from the brain (Fig. 105). A few of the fibers cross the median plane
and join the corresponding root bundles of the opposite side.
The accessory nerve consists of a bulbar and a spinal portion. The fibers of the spinal
root take origin from a linear group of cells in the lateral part of the anterior gray column
in the upper cervical segments of the spinal cord. This root ascends along the side of the
spinal cord, passes through the foramen magnum, and is joined by the bulbar rootlets of the
accessory (Fig. 125). The nerve then divides into an internal and an external branch. In
the latter run all the fibers of spinal origin and these are distributed to the trapezius and
sternocleidomastoid muscles. If, as seems probable, these muscles are derived from the
branchial arches (Lewis, 1910), the fibers which supply them may be regarded as special
visceral efferent fibers; and the spinal nucleus of the accessory nerve may be considered as
homologous to the nucleus ambiguus. The bulbar rootlets of the accessory nerve, which con-
tain both general and special visceral efferent fibers, form a well-defined fascicle, readily
distinguished from the spinal portion of the nerve, which, as the internal ramus, joins (he
vagus nerve and is distributed through its branches (Fig. 120 Chase and Ranson, 1914).
The sensory collaterals which arborize among the cells of the nucleus am-
biguus are derived from the central tracts of the trigeminal, glossopharyngeal,
and vagus nerves, from ascending sensory fibers of spinal origin, and from other
longitudinal fibers in the reticular formation. Other fibers reach this nucleus
from the corticobulbar tract.
THE GENERAL VISCERAL EFFERENT COLUMN
The general visceral efferent column of nuclei is composed of the cells from
which arise the efferent fibers innervating cardiac and smooth muscle and glan-
- ' O^S^A * iaS
JP
A B
Fig. 126. Two types of motor nerve-cells from medulla oblongata of lemur: A, Cells of the
somatic motor type from the hypoglossal nucleus; B, cells of the visceral efferent type from the
rostral part of the dorsal motor nucleus of the vagus. Toluidin blue stain. (Malone.)
dular tissue. The cells of these nuclei are of small or medium size and their
Nissl bodies are not well developed (Fig. 126). They give rise to the general
178 THE NERVOUS SYSTEM
visceral efferent fibers of the cranial nerves. These are small myelinated fibers,
which end in sympathetic ganglia, where they arborize about sympathetic
cells, the axons of which terminate in smooth or cardiac muscle or in glandular
tissue. The neurons of this series are, therefore, characterized by the fact that
the impulses which they transmit must be relayed by neurons of a second order
before reaching the innervated tissue (Fig. 119). This group of nuclei is indi-
cated by black in Fig. 120 and by fine stipple in Figs. 121 and 122.
The dorsal motor nucleus of the vagus (nucleus vagi dorsalis medialis) has
been noted in the transverse sections through the medulla oblongata (Figs. 99,
103). It lies along the dorsolateral side of the hypoglossal nucleus, subjacent
to the ala cinerea of the rhomboid fossa, and along the side of the central canal
in the closed part of the medulla oblongata. The general visceral efferent fibers,
which arise from the cells in this nucleus, leave the medulla oblongata through
the roots of the vagus and accessory nerves; but those entering the accessory
nerve leave that nerve by its internal ramus and join the vagus (Fig. 120).
Hence all of the fibers from this nucleus are distributed through the branches of
the vagus to the vagal sympathetic plexuses of the thorax and abdomen for the
innervation of the involuntary musculature of the heart, respiratory passages,
esophagus, stomach, and small intestines (Van Gehuchten and Molhant, 1912),
and for the innervation of the pancreas, liver, and other glands.
There are relatively few sensory collaterals reaching the dorsal motor nucleus,
and these come in large part from sensory fibers of the second order, arising in
the receptive nuclei of the trigeminal, glossopharyngeal, and vagus nerves.
The nucleus salivatorius is located in the reticular formation, some distance
from the floor of the fourth ventricle at the junction of the pons and medulla
oblongata near the caudal end of the facial nucleus and the rostral end of the
nucleus ambiguus (Figs. 121, 122). The more caudal portion, or nucleus sal-
ivatorius inferior, sends general visceral efferent fibers by way of the glosso-
pharyngeal nerve to the otic ganglion for the innervation of the parotid gland.
The rostral part, or nucleus salivatorius superior, lies dorsal to the large motor
nucleus of the facial nerve, to which nerve it sends general visceral efferent
fibers. These run from the facial nerve through the chorda tympani to the sub-
maxillary ganglion for the innervation of the submaxillary and sublingual sal-
ivary glands (Kohnstamm, 1902, 1903, 1907; Yagita, 1909; Feiling, 1913).
The Edinger-Westphal nucleus is a group of small nerve-cells located in
the rostral part of the nucleus of the oculomotor nerve. Here it is placed
dorsolateral to the median unpaired portion of that nucleus (Figs. 121-123).
THE CRANIAL NERVES AND THEIR NUCLEI 1 70
This group of small cells gives origin to the general visceral efferent fibers of the
oculomotor nerve which run to the ciliary ganglion for the innervation of the
intrinsic muscle of the eye.
Neurobiotaxis. The position of the motor nuclei of the brain stem varies greatly in
different orders of vertebrates, and is determined by the source of the principal afferent
impulses which reach them. The perikarya of the neurons migrate under the influence of an
attraction, which has been called neurobiotaxis, in the direction of the chief fiber tracts
from which they receive impulses (Ariens Kappers, 1914, 1917; Black, 1917). "When from
different places stimuli proceed to a cell, its chief dendrite grows out and its cell body shifts in
the direction whence the majority of the stimuli proceed," while the axon grows in the op-
posite direction (Fig. 127). The nature of the attractive force is not altogether clear. Kap-
Cell
Axiscyh
B
Fig. 127. Diagram to illustrate the principle of neurobiotaxis. The axis-cylinder grows in
the direction of the nervous current, indicated by the arrow, while the dendritic outgrowth and
the final shifting of the cell body occur against the nervous current : A , Dendrites grown out to-
ward the center of stimulation; B, the cell body has shifted toward the center of stimulation; the
axis-cylinder is consequently elongated. (Kappers.)
pers believes that it is a galvanotropic phenomenon, on the basis of the fact that the stimu-
lation center is electrically negative, i. e., a cathode with reference to the surrounding tissue.
Numerous instances might be cited of the action of this taxis, but two will suffice. It
has already been noted that the eye-muscle nuclei receive most of their collaterals from the
optic and vestibular reflex tracts; and these appear to be the most important factors in the
determination of the positions occupied by those nuclei. The changes in position of the nuclei
in the vertebrate series appear to run parallel to the changes in these tracts. The reader
will now appreciate the significance of the close relation of these nuclei to the medial longi-
tudinal and tectospinal fasciculi which convey to them impulses from the vestibular and
optic centers.
The position of the nucleus of the facial nerve and the curved course of its fibers within
the pons may be explained in the same way. In a 10 mm. human embryo the nucleus of the
facial nerve lies rostral to that of the abducens and the motor fibers pass directly lateralward
i8o
THE NERVOUS SYSTEM
to their exit from the brain (Fig. 128). This nucleus, which supplies the muscles that sur-
round the mouth, receives axons from the primary taste center in the medulla oblongata
(the nucleus of the tractus solitarius) which is located at a more caudal level. Accordingly,
the facial nucleus migrates caudally toward that center. It also receives fibers from the
nucleus of the spinal tract of the trigeminal nerve and migrates ventrolaterally toward it.
Thus is explained the adult position of the nucleus of the facial nerve, not far from the
spinal tract of the trigeminal nerve and near the rostral end of the nucleus of the tractus
solitarius. In the same way the curved course of the facial nerve within the pons may be
explained. These examples are perhaps sufficient to illustrate the general principle of neuro-
biotaxis.
Nuclei of Origin and Terminal Nuclei. The efferent nuclei, which we have
examined, all have this in common, that the axons, which take origin from their
constituent cells, leave the brain through the efferent roots of the cranial nerves.
Hence they may all be included under the term nuclei of origin. On the other
hand, the afferent fibers of the cerebrospinal nerves have their cells of origin located
Genu internum n. facialis
Sulcus
Sulcus
Sulcus
Fig. 128. Diagram illustrating three stages in the development of the genu of the facial
nerve, the youngest, A, corresponding to the 10 mm. embryo, and the oldest, C, the newborn
child. The relative position of the nucleus of the n. abducens is represented in outline. Sulcus,
Sulcus medianus fossae rhomboideae. (Streeter, in Keibel and Mall's Embryology.)
outside the central nervous system and, with the exception of the first two cranial
nerves, in the cerebrospinal ganglia. These fibers enter the central nervous
system and end by entering into synaptic relations with sensory neurons of the
second order located in terminal nuclei. These are classified according to the
function of the fibers which end in them as visceral afferent and somatic afferent
nuclei.
THE VISCERAL AFFERENT COLUMN
All of the visceral afferent fibers of the cranial nerves, except those of the first
pair, are contained in the facial, glossopharyngeal, and vagus nerves. These
include: (1) the fibers from the taste buds, which since they mediate the special
sense of taste, may be called special visceral afferent fibers; as well as (2) others
from the posterior part of the tongue, and from the pharynx, larynx, trachea,
esophagus, and thoracic and abdominal viscera, which are known as general
THE CRANIAL NERVES AND THEIR NUCLEI
181
visceral afferent fibers. The majority of the taste fibers run through the seventh
(via the chorda tympani and lingual) and ninth nerves (Gushing, 1903), but a
few reach the epiglottis by way of the tenth (Wilson, 1905 Fig. 129). All
cf these general and special visceral afferent fibers, whether contained in the
seventh, ninth, or tenth nerves, enter the tractus solilarius, within which they
descend for varying distances (Fig. 120, yellow). They terminate in a column
of nerve-cells, which in part surround the tract and in part are scattered among
its fibers. This is known as the nucleus of the tractus solitarius (Figs. 121, 130).
It is a long slender nucleus, which extends throughout the entire length of the
medulla oblongata and is best developed at the level of origin of the vagus nerve,
Fig. 129. Diagram of the trigeminal, facial, and glossopharyngeal nerves showing the course
of the taste fibers in solid black lines. The broken and dotted lines indicate the course which ac-
cording to certain investigators some of the taste fibers are supposed to take: G. G., Gasserian
ganglion; G. g., geniculate ganglion; G. sp., sphenopalatine ganglion; g.s.p., great superficial petro-
sal nerve; N. Jac., the tympanic nerve of Jacobson; N. vid., vidian nerve; s.s.p., small superficial
petrosal nerve. (Gushing.)
where it lies ventrolateral to the dorsal motor nucleus of that nerve and some
little distance below the floor of the fourth ventricle (Fig. 103). The fibers
from the seventh and ninth nerves terminate in the rostral portion of the
nucleus, which is therefore the part especially concerned with the sense of taste,
while those from the vagus end in the caudal part. Some of these vagus fibers
after undergoing a partial decussation terminate in a cell mass, the commissural
nucleus, which lies dorsal to the central canal in the closed part of the medulla
and unites the nucleus of the tractus solitarius on one side with the correspond-
ing nucleus on the other side (Fig. 121).
The secondary afferent paths from the nucleus of the tractus solitarius are
not well denned. Since gustatory impulses arouse sensations of taste and the
182 THE NERVOUS SYSTEM ^
afferent impulses from the viscera may be vaguely represented in conscious-
ness, there must be a visceral afferent path to the thalamus; but concerning
the character and location of this path we are entirely ignorant. 1 The fibers
arising from the nucleus of the tractus solitarius enter the reticular formation,
and it is probable that a majority of them are distributed to the visceral motor
nuclei of the medulla oblongata, including the nucleus ambiguus and the dorsal
motor nucleus of the vagus. In this way arcs are established for a large and
important group of visceral reflexes. Some of these fibers descend to the spinal
cord and may play an important part in the reflex control of respiration and
in initiating reflex coughing and vomiting (Figs. 245, 246).
THE GENERAL SOMATIC AFFERENT NUCLEI
The general somatic afferent nuclei receive fibers from the skin and ecto-
dermal mucous membrane of the head by way of the trigeminal nerve. These
have their cells of origin in the semilunar ganglion, and within the pons the)'
divide into short ascending and long descending branches (Fig. 131). The as-
cending branches terminate in the main sensory nucleus; the descending branches
run through the spinal tract and terminate in the nucleus of the spinal tract of
the trigeminal nerve. Since these nuclei receive sensory fibers from the skin
and ectodermal mucous membrane of the head, they are exteroceptive in function.
The spinal tract and its nucleus also receives a few cutaneous afferent fibers
through the glossopharyngeal and vagus nerves from the skin of the external ear
(Fig. 120).
The main sensory nucleus of the trigeminal nerve is located at the level of
the middle of the pons in the lateral part of the reticular formation some dis-
tance from the floor of the fourth ventricle (Figs. 110, 121, 130). The spinal
nucleus, with which it is continuous, at first lies deeply under cover of the resti-
form body; but when it is traced caudally it approaches the surface and, covered
by the spinal tract, forms the tuberculum cinereum (Figs. 99, 103). It finally
becomes continuous with the substantia gelatinosa Rolandi of the spinal cord.
Thus we have a continuous column of gray matter extending from the sacral por-
tion of the spinal cord into the brain stem and ending abruptly in an enlarge-
ment, the main sensory nucleus of the trigeminal nerve. This entire column
receives afferent fibers from the skin and belongs to the exteroceptive portion of
the somatic afferent division of the nervous system.
1 Kohnstamm and Hindelang (1910) and von Monakow (1913) have described a secondary
visceral afferent path which arises from the gray matter in and around the tractus solitarius and
terminates in the thalamus.
THE CRANIAL NERVES AND THEIR NUCLEI
183
Secondary Afferent Paths. From the cells of the main sensory and spinal
nuclei of the trigeminal nerve arise fibers which enter the reticular formation
and are there grouped into longitudinal bundles from which collaterals are given
off to the motor nuclei of the brain stem (Fig. 131). There are at least two such
longitudinal bundles in each lateral half of the brain. The ventral secondary
afferent path of the trigeminal nerve consists for the most part of crossed fibers
and is located in the ventral part of the reticular formation, close to the spino-
thalamic tract in the medulla, and dorsal to the medial lemniscus in the pons
Mesencephalon \-
Pons-\--
Ventral cochlear nucleus
Medulla oblongata
Cerebral aqueduct
-/ Inferior colliculus
Mesencephalic nucleus of N. V
Sensory nucleus of N. V
-/^-Fourth ventricle
Vestibular nucleus
Dorsal cochlear nucleus
-Nucleus of tractus solitarius
Nucleus of spinal tract N. V
Central canal
*p& M
Fig. 130. Sensory nuclei projected upon a median sagittal section of the human brain stem.
Horizontal lines, general somatic sensory nuclei; cross-hatching, visceral sensory nucleus; stipple,
special somatic sensory nuclei.
and mesencephalon (Fig. 132). It is composed in large part of long fibers which
reach the thalamus. The dorsal secondary afferent path of the trigeminal nerve
consists chiefly of uncrossed fibers and lies not far from the floor of the fourth
ventricle and the central gray matter of the cerebral aqueduct. It consists in
considerable part of short fibers (CajaL 1911; Wallenberg, 1905; Economo,
1911; Dejerine, 1914).
The proprioceptive nuclei of the cranial nerves are not well known. They
have to do with afferent impulses arising in the muscles of mastication and in
1 84
THE NERVOUS SYSTEM
the extrinsic muscles of the eye. Johnston (1909) has shown that the large
unipolar cells of the mesencephalic nucleus of the fifth neroe which give rise
Fig. 131. Diagram of the nuclei and central connections of the trigeminal nerve: A, Semi-
lunar ganglion; B, mesencephalic nucleus, N. V.; C, motor nucleus, N. V.; D, motor nucleus, N.
VII E, motor nucleus, N. XII ; F, nucleus of the spinal tract of N. V. ; G, sensory fibers of the sec-
ond order of the trigeminal path: a, ascending and b, descending branches of the sensory fibers,
N. V.; c, ophthalmic nerve; d, maxillary nerve; e, mandibular nerve. (Cajal.)
to the fibers of the mesencephalic root of that nerve, are probably sensory in
function. Willems (1911) and Allen (1919) believe that these are sensory fibers
THE CRANIAL NERVES AND THEIR NUCLEI
185
to the muscles of mastication. If this interpretation is correct we are pre-
sented with an exception to the rule that the afferent fibers of the cerebrospinal
nerves take origin from cells located outside the cerebrospinal axis. This nucleus
lies in the lateral wall of the rostral portion of the fourth ventricle and in the
lateral part of the gray matter surrounding the cerebral aqueduct (Figs. 114,
121, 130). The origin and termination of the afferent fibers for the extrinsic
Fig. 132. Diagram to show the location of the secondary sensory tracts of the trigeminal
nerve (solid black) in the tegmental portion of the rostral part of the pons: B.C., Brachium con-
junctivum; D.T.T.N., dorsal secondary sensory tract of the trigeminal nerve; L.L., lateral lemnis-
cus; M. L., medial lemniscus; M.L.F., medial longitudinal fasciculus; V.T.T.N., ventral secondary
sensory tract of trigeminal nerve.
muscles of the eye are unknown, although we know that such afferent fibers
are present in the oculomotor, trochlear, and abducens nerves.
SPECIAL SOMATIC AFFERENT NUCLEI
The special somatic afferent nuclei are associated with the acoustic nerve,
which is composed of two divisions. One part, the cochlear neroe, conveys im-
pulses aroused by sound waves reaching the cochlea through the outer ear
and tympanic cavity. Since it responds to stimuli from without, the cochlear
apparatus subserves exteroceptive functions. The vestibular neroe, on the other
hand, conveys impulses from the semicircular canals of the ear. These are im-
portant proprioceptive sense organs and give information concerning the move-
ments and posture of the head.
The cochlear nuclei are the terminal nuclei of the cochlear nerve, the fibers
of which take origin in the spiral ganglion of the cochlea. This is composed of
bipolar cells, each having a short peripheral and a longer central process (Fig.
133). The peripheral process terminates in the spiral organ of Corti. The
central process is directed toward the brain in the cochlear nerve. These central
fibers terminate in two masses of gray matter, located on the restiform body near
the point where the latter turns dorsally into the cerebellum (Figs. 107, 121,
i86
THE NERVOUS SYSTEM
130). One of these masses, the dorsal cochlear nucleus, is placed on the dorso-
lateral aspect of the restiform body and produces a prominent elevation on the
surface of the brain (Fig. 91). The other, known as the ventral cochlear nucleus,
is in contact with the ventrolateral aspect of the restiform body.
Secondary Auditory Path. From the cells of the ventral cochlear nucleus
arise fibers which stream medialward in the ventral part of the pars dorsalis
pontis and form the trapezoid body (Figs. 108, 134). The fibers cross the median
plane and on reaching the lateral border of the opposite superior olivary nucleus
turn rostrally as a compact bundle known as the lateral lemniscus (Figs. 110,
Fig. 133. Section of the spiral ganglion and organ of Corti of the mouse: A, Bipolar cells of
the spiral ganglion; B, outer hair cells; C, sustentacular cells; D, terminal arborization of the
peripheral branch of a bipolar cell about an inner hair cell; T, tectorial membrane. Golgi method.
(Cajal.)
112, 114). Some of the fibers of the trapezoid body end in the superior olivary
nuclei and in the nuclei of the trapezoid body, while others give off collaterals to
these nuclear masses. Some of the fibers arising in these nuclei, especially in
the nuclei of the trapezoid body, join in the formation of the lateral lemniscus;
but according to Cajal (1909) a majority of the fibers from the superior olivary
nucleus belong to short reflex pathways in the reticular formation connecting
the cochlear nerve with the nuclei of the motor nerves of the head and neck.
Fibers arising in the dorsal cochlear nucleus, and possibly also some from the
ventral cochlear nucleus, sweep over the dorsal surface of the restiform body
and the floor of the fourth ventricle as the s trice medullares acusticce. These may
THE CRANIAL NERVES AND THEIR NUCLEI
i8 7
lie just beneath the ependyma or may be buried in the gray matter of the rhom-
boid fossa. On reaching the median plane these fibers decussate, sink into the
reticular formation, and join the trapezoid body or lateral lemniscus of the
opposite side. Some probably fail to cross, since clinical experience and evi-
dence based on animal experiments tend to show that a part of the fibers in the
lateral lemniscus represent an uncrossed path from the cochlear nuclei of the
same side (Kreidl, 1914).
Transverse temporal gyrus
Auditory radiation
Medial geniculate body
Inferior cotticulus
-^"Lateral lemnisci
Collaterals to nucleus of
lateral lemniscus
Rostral portion of the pons-
Caudal portion of the pans
Superior olive-''
/Stria medullares
, Dorsal cochlear nucleus
-Ventral cochlear micleus
Cochlear nene
> Vestibular nerve
Trapezoid body '
Nucleus of the trapezoid body
Fig. 134. Diagram of the auditory pathway. (Based on the researches of Cajal and Kreidl.)
As the lateral lemniscus ascends in the reticular formation of the pons, there
are scattered among its fibers many nerve-cells which together constitute the
nucleus of the lateral lemniscus. To these cells it gives off collaterals and pos-
sibly also terminal branches, and from them it is said to receive additional fibers.
But according to Cajal the axons arising here do not ascend in the lateral lem-
niscus, but are directed medially into the reticular formation.
On reaching the mesencephalon the lateral lemniscus terminates in part in
the inferior cotticulus, but also sends branches and direct fibers by way of the
inferior quadrigeminal brachium to the medial geniculate body. While the me-
i88
THE NERVOUS SYSTEM
dial geniculate body is a way-station on the auditory path to the cerebral cor-
tex, the inferior colliculus serves as a center for reflexes in response to sound.
The Vestibular Nuclei. The fibers of the vestibular nerve take origin from
the bipolar cells of the vestibular ganglion located in the internal auditory meatus
(Fig. 135). The cochlear and vestibular divisions of the acoustic nerve sepa-
rate at the ventral border of the restiform body. Here the vestibular nerve
Fig. 135. The vestibular ganglion and the termination of the peripheral branches of its bi-
polar cells in a macula acustica: A, Hair cells and B, sustentacular cells of the macula; D, terminal
arborization of the peripheral branches of the bipolar cells of the vestibular ganglion (G) about the
hair cells of the macula; F, facial nerve; R, central branches of the bipolar cells directed toward the
medulla oblongata T. Mouse. Golgi method. (Cajal.)
penetrates into the brain, passing between the restiform body and the spinal
tract of the trigeminal nerve toward the area acustica of the rhomboid fossa.
Under cover of the area acustica the fibers divide into short ascending and
longer descending branches (Figs. 134, 136). There may be enumerated five
cellular masses within which these fibers terminate, namely: (1) the principal
or medial nucleus, (2) the descending or spinal nucleus, (3) the superior nucleus
THE CRANIAL NERVES AND THEIR NUCLEI
189
of Bechterew, (4) the lateral nucleus of Deiters, and (5) the cerebellum (Figs.
130, 136).
The principal, medial, or dorsal vestibular nucleus is very large. It lies sub-
jacent to the major portion of the area acustica and belongs, therefore, to both
the pons and the medulla oblongata (Figs. 89, 103, 107). The gray matter,
associated with the descending branches from the vestibular nerve and lying on
the medial side of the restiform body, constitutes the spinal or descending
vestibular nucleus. Along with the descending fibers it can be followed in serial
Nuc. of oculomotor _
neroe
Nuc. of trocMear nerve -^-
Brachium
Nuc. of abducens nerve.
Rhomboid fossa'
Medulla oblongata-
Superior colliculus
Inferior colliculus
Med. longitudinal
- " ' fasciculus
Superior vestibular
Vestibulocerebellar
tract
4 Lateral vestibular nuc.
" Vestibular nerve
' Spinal vestibular nuc.
Principal vestibular
nucleus
Fig. 136. Diagram of the nuclei and central connections of the vestibular nerve. (Based on
figures by Herrick and Weed.)
sections as far as the rostral extremity of the nucleus gracilis. The lateral vestib-
ular nucleus of Deiters is situated close to the restiform body at the point where
the fibers of the vestibular nerve begin to diverge (Fig. 107). It is composed of
large multipolar cells like those found in motor nuclei. Directly continuous with
the medial and lateral nuclei is a mass of medium-sized cells, the superior vestib-
ular nucleus of Bechterew, located in the floor and lateral wall of the fourth
ventricle lateral to the abducens nucleus, and the emergent fibers of the facial
nerve (Fig. 108). It extends as far rostrad as the caudal border of the main
sensory nucleus of the trigeminal nerve (Weed, 1914).
190 THE NERVOUS SYSTEM
Many of the ascending branches of the vestibular nerve, after giving off
collaterals to the nuclei of Deiters and Bechterew, are prolonged in the tractus
vestibulocerebellaris, to end in the cortex of the cerebellum (Cajal, 1909). These
are joined by fibers arising in the superior and lateral vestibular nuclei which
also run to the cerebellum (Fig. 136). From the standpoint of its embryologic
development the cerebellum may properly be regarded as a highly specialized
vestibular nucleus (p. 196).
Secondary Vestibular Paths. In addition to the fibers to the cerebellum
mentioned in the preceding paragraph two important tracts of fibers take origin
in the superior and lateral vestibular nuclei. One of these was encountered in
the study of the medial longitudinal bundle. Cells in the superior and lateral
vestibular nuclei give rise to fibers which run to the medial longitudinal fascicle
of the same and of the opposite side, and through it reach the motor nuclei of the
ocular muscles (Fig. 136). In this way there is established an arc, which makes
possible the reflex response of the eye muscles to afferent impulses arising in the
vestibule and semicircular canals of the ear. The other bundle was considered
in connection with the spinal cord as the vestibule spinal tract, the fibers of
which take origin from the cells of the lateral nucleus and descend into the
anterior funiculus of the same side of the cord. These fibers serve to place the
primary motor neurons of the spinal cord under the reflex control of the vestib-
ular apparatus.
From the medial border of the principal vestibular nucleus many scattered
fibers cross the raphe and enter the reticular formation of the opposite side, where
they become longitudinal fibers. No tract to the thalamus is known, a fact
which is in keeping with this other, that ordinarily the activities of the vestib-
ular apparatus are not clearly represented in consciousness.
SUMMARY OF THE ORIGIN, COMPOSITION, AND CONNECTIONS OF THE CRANIAL
NERVES
The olfactory and optic nerves and the nervus terminalis, which have not
yet been considered in detail, have been included in this summary for the sake
of completeness.
The nervus terminalis is a recently discovered nerve which arises from the
cerebral hemisphere in the region of the medial olfactory tract or stria. It is
closely associated with the olfactory nerve and its fibers run to the nasal septum.
The origin, termination, and function of its component fibers are not yet under-
stood (McKibben, 1911; Huber and Guild, 1913; McCotter, 1913; Johnston,
THE CRANIAL NERVES AND THEIR NUCLEI 191
1914; Brookover, 1914, 1917; Larsell, 1918, 1919). Since it was unknown at
the time the cranial nerves were first enumerated, it bears no numerical desig-
nation.
I. Olfactory Nerve. Superficial origin from the olfactory bulb in the form of
a number of fine fila which separately pass through the openings in the cribri-
form plate. It is composed of special visceral afferent fibers with cells of origin
in the olfactory mucous membrane. The fibers terminate in the glomeruli of
the olfactory bulb.
II. Optic Nerve. Not a true nerve; but both from the standpoint of its
structure and development a fiber tract of the brain. Superficial origin, from the
optic chiasma, or after partial decussation, from the lateral geniculate body,
pulvinar of the thalamus, and superior colliculus. Component fibers: special
somatic afferent exteroceptive; origin, ganglion cells of the retina; terminations
in the lateral geniculate body, pulvinar of the thalamus and superior colliculus.
The fibers from the nasal half of each retina cross in the optic chiasma. 1
III. Oculomotor Nerve. Superficial origin, from the oculomotor sulcus on
the medial aspect of the cerebral peduncle. Composition:
1. Somatic Efferent Fibers. Cells of origin, hi the oculomotor nucleus of the
same and to a less extent of the opposite side (Fig. 120). Termination, in the
extrinsic muscles of the eye except the superior oblique and the lateral rectus.
2. General Visceral Efferent Fibers. Cells of origin in the Edinger-Westphal
nucleus. Termination in the ciliary ganglion, from the cells of which post-
ganglionic fibers run to the intrinsic nuscles of the eye. 2
IV. Trochlear Nerve. Superficial origin, from the anterior medullary ve-
lum. Composed of somatic efferent fibers; cells of origin in the trochlear nucleus;
decussation in the anterior medullary velum; termination in the superior oblique
muscle of the eye (Fig. 120).
V. Trigeminal Nerve. Superficial origin, from the lateral aspect of the
middle of the pons by two roots: the portio major or sensory root and the portio
minor or motor root. Composition (Fig. 120):
1. General Somatic A/erent Fibers. A, Exteroceptive Cells of origin in the
semilunar ganglion (Gasserii), chiefly unipolar with T-shaped axons, peripheral
1 It has been demonstrated by Arey that there are also efferent fibers in the optic nerves of
fishes which control the movement of the retinal elements in response to light, Jour. Comp. Neur.,
vol. 26, p. 213.
2 It is probable that the oculomotor, trochlear, and abducens nerves contain proprioceptive
fibers for the extrinsic muscles of the eye, but the cells of origin and the central connections of
these sensory components are unknown.
IQ2 THE NERVOUS SYSTEM
branches to skin and mucous membrane of the head, central branches by way
of the portio major to the brain. Termination in the main sensory nucleus and
nucleus of the spinal tract of the trigeminal nerve.
2. General Somatic Afferent Fiber's. B, Proprioceptive Cells of origin prob-
ably located in the mesencephalic nucleus of the fifth nerve. Fibers by way
of the portio major, distributed as sensory fibers to the muscles of mastication.
3. Special Visceral Efferent Fibers. Cells of origin in the motor nucleus of
the fifth nerve. Fibers by way of the portio minor and the mandibular nerve
to the muscles of mastication.
VI. Abducens Nerve. Superficial origin, from the lower border of the
pons just rostral to the pyramid. Composed of somatic efferent fibers; cells of
origin in the abducens nucleus; termination in the lateral rectus muscle of the
eye.
VII. Facial Nerve and Nervus Intermedius. Superficial origin from the
lateral part of the lower border of the pons separated from the flocculus by the
eighth nerve. Composition (Fig. 120) :
1. Special Visceral Afferent Fibers. Cells of origin in the ganglion geniculi,
chiefly unipolar, with T-shaped axons. The peripheral branches run by way of
the chorda tympani and lingual nerves to the taste buds of the anterior two-
thirds of the tongue. The central branches run by way of the nervus intermedius
to the tractus solitarius and end in the nucleus of that tract. It is probable that
the taste fibers terminate in the rostral part of this nucleus. 1
2. General Visceral Efferent Fibers. Cells of origin in the nucleus saliva torius
superior. These fibers run by way of the- nervus intermedius, facial nerve,
chorda tympani, and lingual nerve to the submaxillary ganglion for the in-
nervation of the submaxillary and sublingual salivary glands.
3. Special Visceral Efferent Fibers. Cells of origin in the motor nucleus of
the facial nerve. These fibers run by way of the facial nerve to end in the super-
ficial musculature of the face and scalp, and in the platysma, posterior belly of
the digastric, and stylohyoid muscles.
VIII. Acoustic Nerve. Superficial origin from the lateral part of the lower
border of the pons near the flocculus. Consists of two separate parts known as
the vestibular and cochlear nerves.
1 Herrick (1918) describes general visceral afferent fibers in the facial nerve which he says
mediate deep visceral sensibility and are probably found in all the branches of the facial. And
Rhinehart (1918) has described a cutaneous branch of the facial in the mouse. This branch con-
tains general somatic afferent fibers, which arise in the geniculate ganglion and terminate in the
skin.
THE CRANIAL NERVES AND THEIR NUCLEI 193
The Vestibular Nerve. The component fibers belong to the special somatic
afferent group and are proprioceptive. Cells of origin, in the vestibular ganglion,
are bipolar. Their peripheral branches run to the semicircular canals, utricle and
saccule. Their central branches terminate in the principal, lateral, superior, and
spinal vestibular nuclei. Some of them run without interruption to the cerebellum.
The Cochlear Nerve. The component fibers belong to the special somatic
afferent group and are exteroceptive. Cells of origin, in the spiral ganglion of
the cochlea, are bipolar. Their peripheral branches end in the spiral organ of
Corti. Their central branches terminate in the ventral and dorsal cochlear nuclei.
IX. The Glossopharyngeal Nerve. Superficial origin, from the rostral
end of the posterior lateral sulcus of the medulla oblongata in line with the
tenth and eleventh nerves. Composition (Fig. 120):
1. General Visceral Afferent Fibers. Cells of origin in the ganglion petrosum,
peripheral branches form the general sensory fibers to the pharynx and posterior
third of the tongue, central branches run to the tractus solitarius and its nucleus.
2. Special Visceral Afferent Fibers. Cells of origin in the ganglion petrosum,
peripheral branches to the taste buds of the posterior third of the tongue, central
branches, to the tractus solitarius and its nucleus.
3. General Visceral Efferent Fibers. Cells of origin in the inferior salivatory
nucleus; fibers run to the otic ganglion, from the cells of which postganglionic
fibers carry the impulses to the parotid gland.
4. Special Visceral Efferent Fibers. Cells of origin in the nucleus ambiguus.
Termination in the stylopharyngeus muscle.
X. Vagus Nerve. Superficial origin from the rostral part of the posterior
lateral sulcus of the medulla oblongata in line with the ninth and eleventh and
just caudal to the ninth. Composition (Fig. 120) :
1. General Somatic Afferent Fibers. Cells of origin in the ganglion jugulare;
peripheral branches to the skin of the external ear by way of the ramus auricularis;
central branches to the spinal tract of the trigeminal nerve and its nucleus.
According to Herrick, some of these fibers from the external ear run by way of
the glossopharyngeal nerve also.
2. General Visceral Afferent Fibers. Cells of origin in the ganglion nodosum;
peripheral branches run as sensory fibers to the pharynx, larynx, trachea, esopha-
gus, and the thoracic and abdominal viscera ; central branches run to the tractus
solitarius and terminate in its nucleus. 1
According to Wilson (1905) there are also special visceral afferent fibers in the vagus for
the taste buds of the epiglottis. These also terminate in the nucleus of the tractus solitarius.
13
194 THE NERVOUS SYSTEM
3. General Visceral Efferent Fibers. Cells of origin in the dorsal motor nucleus
of the vagus. Fibers run to the sympathetic ganglia of the vagal plexuses for
the innervation of the thoracic and abdominal viscera.
4. Special Visceral Efferent Fibers. Cells of origin in the nucleus ambiguus.
Termination in the striated musculature of the pharynx and larynx.
XI. Accessory Nerve. Superficial origin from the posterior lateral sulcus
of the medulla oblongata caudal to the ninth and tenth and from the lateral as-
pect of the first five or six cervical segments of the spinal cord. Composition
(Fig. 120):
1. General Visceral Efferent Fibers. Cells of origin in the dorsal motor
nucleus of the vagus. Fibers run in the bulbar rootlets and then by way of the
internal ramus of the accessory to join the vagus, and end in the sympathetic
plexuses, associated with the vagus nerve, for the innervation of thoracic and
abdominal viscera.
2. Special Visceral Efferent Fibers. These fall into two groups: A, fibers,
whose cells of origin are located in the nucleus ambiguus, and which run by way of
the internal ramus of the accessory to join the vagus and are distributed through
it to the striated muscles of the pharynx and larynx; B, fibers, whose cells of
origin lie in the lateral part of the anterior gray column of the first five or six
cervical segments of the spinal cord, and which ascend in the spinal root of the
accessory nerve and then run in its external ramus to end in the trapezius and
the sternocleidomastoid muscles.
XII. Hypoglossal Nerve. Superficial origin from the anterior lateral sulcus
of the medulla between the pyramid and the olive. It is composed of somatic
efferent fibers, whose cells of origin are located in the hypoglossal nucleus and
whose termination is in the musculature of the tongue.
CHAPTER XIII
THE CEREBELLUM
DEVELOPMENT OF THE CEREBELLUM
THE dorsal border of the alar lamina occupies a lateral position in the rhom-
bencephalon and, as a result of the development of the pontine flexure, acquires
a V-shaped bend at the apex of which is the lateral recess of the fourth ventricle
(Fig. 137, A). This dorsal border becomes everted and forms a prominent
Mid-brain
Cerebellum
Lateral recess
Rhombic lip
Corpora quadrigemina
Cerebrum
A nlage of
vermis
Lateral lobe of
cerebellum
Rhombic lip
Lateral lobe of cerebellum Lobules of vermis
Obex
Flocculus
Uvula
Nodulus
Fig. 137. Dorsal view of four stages in the development of the cerebellum: A, of a 13.6
mm. embryo (His); B, of a 24 mm. embryo; C, of a 110 mm. fetus; D, of a 150 mm. fetus. (Pren-
tiss and Arey.)
ridge known as the rhombic lip. From the portion of this ridge caudal to the
lateral recess develop the taenia of the fourth ventricle and the obex. At the
level of the recess the fibers of the acoustic nerve reach the dorsal edge of the
alar lamina, which, accordingly, undergoes development at this point into
vestibular and cochlear nuclei. More rostrally it undergoes an excessive devel-
195
196 THE NERVOUS SYSTEM
opment, which is stimulated by the growth into it of afferent fibers from the
vestibular nerve and of sensory fibers of the second order, bringing afferent
impulses from other sources, chiefly from the somatic musculature. This
part of the alar lamina, which may be regarded as an overgrown portion of the
vestibular nucleus, develops into the cerebellum. As the paired cerebellar plates
increase in thickness during the second month of embryonic development, they
bulge inward toward the ventricle and take up a transverse position (Fig. 137,
5). As they increase in size they invade the roof plate and unite in the median
plane forming a transverse bar above the fourth ventricle. The lateral ex-
tremities of this bar expand, and the entire structure assumes a dumb-bell
shape, the lateral masses representing the future cerebellar hemispheres and the
intermediate part the future vermis.
At the close of the third month transverse sulci begin to appear in the vermis.
The first of these, the fissura prima or sulcus primarius, extends into the lateral
masses on either side and separates an anterior lobe from the remainder of the
cerebellum. Other transverse fissures soon appear, due to the rapid expansion
and resultant folding of the cortical layers.
The cerebellum differs from the other parts of the nervous system, which we
have thus far studied in detail, in that the relative position of the gray and white
matter is reversed. The gray substance forms a thin superficial layer, the
cerebellar cortex, which covers a central white medullary body (corpus medullare).
Originally the cerebellar plate is formed, like other parts of the neural tube, of
an ependymal, a nuclear or mantle, and a cell-free marginal zone. The neuro-
blasts of the mantle zone take no part in the formation of the cortex, but become
grouped in the internal nuclear masses of the cerebellum. The superficial or
marginal zone is at first devoid of nuclei; the neuroblasts, from which the cere-
bellar cortex is differentiated, migrate into this zone from the ependymal and
perhaps also from the mantle layers of the rhombic lip. These developing neu-
rons send their axons inward instead of outward as in the case of the spinal cord.
These axons accumulate, along with others which enter the cerebellum from
without, in the deep part of the marginal layer and form the central medullary
body of the cerebellum, separating the developing cortex from the deep nuclear
masses that are differentiating from the mantle layer.
THE ANATOMY OF THE CEREBELLUM
It is customary to consider the cerebellum as composed of three parts: a
small unpaired median portion, called the vermis, because superficially it re-
THE CEREBELLUM
197
sembles a worm bent on itself to form almost a complete circle; and two large
lateral masses, the cerebellar hemispheres, which are connected with each other
by the verrm's (Figs. 138, 139). Although morphologically incorrect, this sub-
division has the advantage of convenience as well as of established usage. On
the rostral aspect of the cerebellum the vermis forms a median ridge, not sharply
marked off laterally from the hemispheres. This part has been called the superior
vermis, and in contradistinction the remainder is known as the inferior vermis.
The latter forms a prominent ridge, marked off from the hemisphere on either
side by a well-defined sulcus. It lies in a deep groove between the hemispheres,
known as the vallecula, within which the medulla oblongata is lodged. The
hemispheres are also partially separated from each other by deep notches, the
Anterior cerebellar notch
Central lobule
f Ala of central lobule
Quadrangu-$ Ant, portion
lar lobule \p os t. portion
Cerebellar hemi-
sphere superior
surface
Superior semi- .
lunar lobule
Cerebellar folia
Inferior semilunar lobule
Primary fissure
Postclival sulcus
Horizontal cerebellar sukus
Folium of vermis
Posterior cerebellar notch
Fig. 138. Dorsal view of the human cerebellum. (Modified from Sobotta-McMurrich.)
incisura cerebelli. The anterior cerebellar notch (semilunar notch) is broad and
deep; and as seen from above it is occupied by the brachia conjunctiva and
the inferior colliculi of the corpora quadrigemina. The posterior cerebellar
notch (marsupial notch) is smaller, and within it is lodged a fold of the dura
mater, the falx cerebelli.
The superior vermis is divided by transverse fissures into the following
lobules (Fig. 138):
1. Lingula, closely applied to the anterior medullary velum between the two
brachia conjunctiva.
2. Central lobule, associated with the small alae lobuli centralis of the hemi-
sphere.
198
THE NERVOUS SYSTEM
3. Monticulus, which is further subdivided into the culmen and declive. The
former goes over laterally without line of demarcation into the anterior portion
of the quadrangular lobule, and the latter into the posterior portion of the same
lobule in the hemisphere.
4. Folium vermis at the posterior extremity of the superior vermis.
The rostral or dorsal surface of the hemisphere is subdivided by curved
transverse fissures, which are continued across the vermis, into the following
parts :
1. The anterior part of the quadrangular lobule, continuous with the culmen
monticuli of the vermis.
2. The posterior part of the quadrangular lobule, continuous with the declive
monticuli.
Nodule of vermis Flocculus
Inferior vermis
Cerebellar hemisphere
inferior surface^--.
Tonsil
Bivenlral lobule
^Inferior semi-
lunar lobule
.- Horizontal cere-
bellar stdcus
Superior semilunar
lobule
Uvula of vermis j p os i er - lor N N Tuber of vermis
Pyramid of vermis ' cerebettar Folium of vermis
notch
Fig. 139. Ventral view of the human cerebellum. (Sobotta-McMurrich.)
3. The superior semilunar lobule, occupying a large crescentic area along the
dorsolateral border of the rostral surface.
The inferior vermis (Fig. 139) is divided by transverse sulci into the follow-
ing lobules:
1. The tuber vermis, next to the folium.
2. The pyramis.
3. The uvula.
4. The nodulus.
The caudal surface of the hemisphere presents the following subdivisions:
1. The inferior semilunar lobule, occupying a large part of this surface along
its dorsolateral border.
THE CEREBELLUM
199
2. The biventral lobule, occupying the ventrolateral part of the inferior surface.
3. The tonsil, a small rounded lobule near the inferior vermis.
4. The flocculus is the smallest of the lobules; and from it there runs toward
the median plane a thin white band, the posterior medullary velum, and the
peduncle of the flocculus.
Structure of the Cerebellum. The cerebellum is composed of a thin super-
ficial lamina of gray matter, spread over an irregular white center that con-
tains several compact nuclear masses. This white medullary body forms a
compact mass in the interior and is continuous from hemisphere to hemisphere
through the vermis, within which, however, it is smaller than in the hemi-
spheres (Figs. 140, 141). As is most readily seen in sagittal sections through the
cerebellum, the medullary body gives off numerous thick laminae, which pro-
Dentate nucleus
Central lobule
Lingula
Fissura prima
Declive
Folium
Nodule Uvula
Fig. 140. Fig. 141.
Figs. 140 and 141. Sagittal sections of the human cerebellum: Fig. 140 passes through the
hemisphere and dentate nucleus; Fig. 141, through the vermis in the median plane.
ject into the lobules of the cerebellum; and from these there are given off sec-
ondary and tertiary laminse at various angles. Thus a very irregular white
mass is formed, over the surface of which the much folded cortex is spread in
a thin but even layer. Supported by the white laminae, the cortex forms long
narrow folds, known as folia, which are separated by sulci and which are aggre-
gated into lobules that, in turn, are separated by more or less deep fissures.
Sections through the cerebellum at right angles to the long axis of the folia thus
present an arborescent appearance to which the name arbor vita has been ap-
plied. This is particularly evident in sections through the vermis (Fig. 141).
MORPHOLOGY OF THE CEREBELLUM
According to Elliott Smith (1903) and Bolk (1906), who have carried out extensive
investigations on the morphology of the mammalian cerebellum, the fissura prima is an
2OO
THE NERVOUS SYSTEM
important and constant fissure. It extends in a continuous curved line across the rostral
aspect. of the vermis and both hemispheres. It has been found by Ingvar (1918) in reptiles
and birds. All investigators who have given attention to this subject in recent years agree
in designating the portion of the cerebellum which lies rostral to the fissura prima as the
anterior lobe. The portion behind this fissure is composed of several individual lobules, each
of which, though subject to considerable variation in form in the different genera, can be
identified in every mammalian cerebellum. These lobules have been variously grouped into
lobes by different investigators. Here we will follow the grouping employed by Ingvar, which
is based on a comparison of the mammalian cerebellum with that of birds and reptiles (Fig.
142). He recognizes three major divisions of the cerebellum, which he designates as the
anterior, middle, and posterior lobes. The middle lobe contains those parts of the cerebellum
which have been the last to appear during phyletic development, and it is here that the
greatest variations are found in the different orders of mammals.
1.
Fig. 142. Schematic drawing of the cerebellum of 1, lizard; 2, crocodile; 3, bird, and 4,
mammal. Vertical lines, anterior lobe; stipple, middle lobe; horizontal lines, posterior lobe; white,
lobus ansoparamedianus. (Ingvar.)
The anterior lobe includes all that part of the cerebellum that lies on the rostral side of
the fissura prima (Figs. 143, 144, 146). In this lobe the folia have a transverse direction and
extend without interruption across the vermis into both hemispheres. In the sheep the an-
terior lobe is bounded laterally by the parafloccular fissure. It includes the three most
rostral lobules of the superior vermis, which are designated in order from before backward, the
lingula, lobulus centralis, and culmen monticuli. In man it also includes a large wing-shaped
portion of each hemisphere (the pars anterior lobuli quadrangularis) ; and the entire lobe has
the shape of a butterfly (Fig. 146). Morphologically, it is a median unpaired structure.
The middle lobe is subdivided into four parts (Fig. 142). The most rostral of these
is the lobulus simplex. It is separated from the anterior lobe by the fissura prima, and like
that lobe it consists of transverse folia which extend across the superior vermis into both
THE CEREBELLUM
201
hemispheres (Figs. 143, 144). In man the lobulus simplex forms a broad crescentic band
across the rostral surface of the cerebellum, including what is ordinarily designated as the
posterior part of the quadrangular lobule and the declive monticuli (Fig. 146). Like the
anterior lobe, it is a median unpaired structure. The remainder of the middle lobe is sub-
divided into median and lateral portions. The median part, known as the tuber vermis
Fissura prima
Lobulus ansiformis
Lobulus paramedianus
Lobits anterior
^ Lobulus simplex
' Paraflocculus
-Fissura parafloccularis
"^ Tuber vermis
Fig. 143. Cerebellum of the sheep, dorsorostral view.
(lobulus medius medianus of Ingvar and lobulus C 2 of Bolk), forms a conspicuous S-shaped
lobule in the vermis of the sheep (Fig. 145) and may be readily identified at the occipital
extremity of the inferior vermis in man (Figs. 139, 141). The paired lateral portions of the
middle lobe each consist of two parts, called the lobulus ansiformis and lobulus paramedianus.
The lobulus ansiformis, relatively small in most mammals (Fig. 144), is very large in man,
Fissura prima
i
i
, Lobus anterior
/ f Lobuhis simplex
Flocculus-"
Paraflocculus'
Lobulus paramedianus '
"Lobulus ansiformis
Tuber vermis
Lobulus medianus posterior
Fig. 144. Cerebellum of the sheep, lateral view.
and forms approximately the dorsolateral half of the hemisphere, occupying considerable
parts of both the rostral and caudal surfaces. It corresponds to what has been known as
the superior and inferior semilunar lobules and the biventral lobule (Figs. 146, 147). The
lobulus paramedianus, or tonsilla of the B. N. A., is located on the lateral surface of the
sheep's cerebellum, but is displaced on to the caudal surface in man by the great expani
of the lobulus ansiformis.
202
THE NERVOUS SYSTEM
The posterior lobe, as outlined by Ingvar, is composed of median and lateral portions.
The median part, known as the posterior median lobule, comprises all of the inferior vermis
except the tuber, from which it is separated by the prepyramidal sulcus. It is subdivided
into three sublobules, known as the nodule, uvula, and pyramid (Figs. 139, 141, 145). The
lateral part of the posterior lobe is formed on either side by two lobules, known as the flocculus
and paraflocculus. These form the most lateral portion of the hemisphere in most mammals
(Figs. 142, 144). In man the paraflocculus is rudimentary and the flocculus lies upon the
caudal surface of the hemispheres (Fig. 147). It is connected with the nodule by a thin
sheet of white matter, the posterior medullary velum.
Functional Localization in the Cerebellum. We have described the cerebellum in
terms of the subdivisions of Bolk and Ingvar, because these have morphologic and physio-
logic significance, which is not true of the parts into which the cerebellum had previously
been divided. By comparison of the size of these subdivisions with the degree of develop-
ment and functional importance of the various groups of muscles in different animals Bolk
endeavored to show that each of these parts was related to a particular group of muscles.
On the basis of these comparative studies he concluded that the median unpaired portions
of the cerebellum serve as coordination centers for the muscles which function in bilateral
Tuber vermis
Prepyramidal snlcus^
Paraflocculus --
-' Lobulus ansiformis
Lobulus paramedianus
~ Lobulus mcdianus posterior
Fig. 145. Cerebellum of the sheep, caudal view.
synergy. The muscles of expression and mastication, those of the eyes, pharynx, larynx
and neck, and many of the trunk muscles are called into action simultaneously on both sides
of the body, and should, according to this theory, have a median unpaired representation
in the cerebellum. Bolk located the coordination center for the musculature of the head
in the anterior lobe, that for the muscles of the neck in the lobulus simplex (Figs. 146, 147).
A median center for those movements of the extremities which are strictly bilateral is found
in the most dorsal sublobule of the vermis inferior, known as lobulus C 2 or tuber vermis.
The remainder of the inferior vermis forms, according to this theory, a center for the bilateral
movements of the trunk. In addition to a median center in the tuber vermis, the limbs are
represented in the cerebellum by lateral centers for the coordination of unilateral move-
ments. The lateral center for the arm is located in the rostral part or crus primum of the
lobulus ansiformis (superior and inferior semilunar lobules) and that for the legs in the caudal
part or crus secundum (biventral lobule), and perhaps also in the lobulus paramedianus
(tonsil) .
The conclusions concerning the localization of function in the cerebellum, reached by
Bolk on the basis of morphologic studies, have been confirmed in so far as the centers for the
neck and extremities are concerned by animal experimentation (Van Rynberk, 1908, 1912;
THE CEREBELLUM
203
Andre Thomas and Durupt, 1914) and by clinical observations (Barany, 1912). There
are, however, good reasons for skepticism regarding his localization of centers for the head
and trunk. Ingvar (1918) presents evidence which indicates that the anterior and posterior
lobes are probably concerned with the maintenance of the equilibrium of the body as a whole.
The middle lobe, on the other hand, contains a number of separate centers, which correspond
to those outlined by Bolk, for the control of the musculature of the neck and extremities.
It has long been known that the degree of development of the cerebellar hemispheres in the
different classes of vertebrates is closely correlated with that of the pons and cerebral cortex.
This is particularly true of the lobulus ansiformis and lobulus paramedianus, which, like the
neopallium, are recent phyletic developments. These belong to what Edinger (1911) calls
B. N. A.
Ala lobuli centralis
Lobulus centralis
Culmen monticuli
Pars anterior lobuli
quadrangular is
Pars posterior lobuli
quadrangularis
Declive monticu'i
Lobulus semilunaris
superior
Lobulus centralis
Ala lobuli centralis
Brachium pontis
Flocculus
Brachium conjunctivum
Nodulus
Uvula
Tonsilla
Lobulus biventer
Pyramis
Tuber
Lob. semilun. inf.
Sulcus horizontalis
Lobulus semilunaris
superior
Fig. 146.
BOLK
Lobus anterior
Sulcus primarius
Lobulus simplex
S. postdivalis
Lobulus ansiformis
Lobus anterior
Cerebellar peduncles (cut)
Flocculus
Sulcus uvulo-nodularis
Lobulus paramedianus
Fissura secunda
Lobulus ansiformis
Fig. 147.
Figs. 146 and 147. Outline drawings of the human cerebellum showing the localization of
function according to the theory of Bolk. On the right side the parts are designated according
to Bolk's terminology, on the left according to the B. N. A. Fig. 146, dorsal view. Fig. 147,
ventral view. (Herrick.)
the neocerebellum, receive the majority of the fibers from the brachium pontis, and may
properly be regarded as cortical dependencies. They take an important part in the co-
ordination of the voluntary movements of the extremities.
THE NUCLEI OF THE CEREBELLUM
The dentate nucleus is a crumpled, purse-like lamina of gray matter within
the massive medullary body of each cerebellar hemisphere (Fig. 148). Like
the inferior olivary nucleus, which it closely resembles, it has a white center
and a medially placed hilus. In close relation to this hilus lies a plate of gray
matter, the emboliform nucleus, and medial to this is the small globose nucleus.
204
THE NERVOUS SYSTEM
Close to the median plane in the medullary body of the vermis, where this forms
the tent-like covering of the fourth ventricle, is the nucleus of the roof or nucleus
fastigii.
The dentate nucleus is well developed only in those animals which possess
large cerebellar hemispheres. It receives fibers from the cortex of the cere-
bellar hemisphere, while the nuclei fastigii and globosi receive fibers chiefly
from the vermis (Clark and Horsley, 1905; Edinger, 1911). It is probable that
Decussation of brachia conjunctiva --, :
Medial longitudinal fasciculus--^'
\"'
Brachium
Molecular layer
Granular layer
Rhomboid fossa
'
^y A nterior medullary velum
Lingula of cerebellum
Fastigial nucleus
Hilus of dentate nucleus
Dentate nucleus
Medullary lamina' 4
Cerebellar folia-'-'
Medullary substance of /'
hemisphere
Emboliform nucleus
Globose nucleus
Vermis
Capsule of dentate nucleus
Posterior cerebellar notch
Fig. 148. Horizontal section through the cerebellum showing the location of the central nuclei.
(Sobotta-McMurrich.)
a functional localization similar to that in the cerebellar cortex will be found
to exist in the central nuclei. In histologic structure the central nuclei closely
resemble the inferior olive.
THE CEREBELLAR PEDUNCLES
The white core of the cerebellum is formed in large part of fibers which enter
and leave the cerebellum through its three peduncles.
The brachium pontis, or middle cerebellar peduncle, is formed by the trans-
verse fibers of the pons and carries impulses which come from the cerebral cortex
of the opposite side. It enters the cerebellum on the lateral side of the other
two, and is distributed in two great bundles: one from the rostral part of the
pons radiates to the caudal part of the cerebellar hemisphere; the other, from the
caudal part of the pons, spreads out to the rostral portion of the hemisphere.
In man, as might be expected from the large size of the pons and cerebellar
THE CEREBELLUM
205
hemispheres, the brachium pontis is the largest of the three peduncles (Fig.
89). But this is not true in most mammals, where, as in the sheep, the cere-
bellum receives the majority of its afferent fibers from the spinal cord and medulla
oblongata by way of the relatively large restiform bodies (Fig. 91).
The restiform body ascends along the lateral border of the fourth ventricle;
and at a point just rostral to the lateral recess it makes a sharp turn dorsally
to enter the cerebellum between the other two peduncles (Figs. 87, 88). It
consists of ascending fibers from the spinal cord and medulla oblongata and prob-
ably also of descending fibers from the cerebellum to the reticular formation
of the medulla (fastigiobulbar tract, p. 211). Among the ascending fibers are
those of the following bundles: (1) dorsal spinocerebellar tract, which arises
/ Tectocerebellar tract
Cerebellum-
Restiform body -
Dorsal spinocerebellar tract * N
Ventral spinocerebellar tract x^
f /
' .; Corpora quadrigemina
/ ''!
Brachiumconjunctivum
-Pons
Fig. 149. Diagram of the spinocerebellar and tectocerebellar tracts.
from the cells of the nucleus dorsalis of the same side of the spinal cord and
ends in the cortex of the vermis; (2) the olivocerebellar tract, which consists of
fibers from the opposite inferior olivary nucleus and to a less extent from that
of the same side and which ends in the cortex of the vermis and of the hemi-
sphere and in the central nuclei; (3) the dorsal external arcuate fibers, from the
nuclei of the posterior funiculi of the same side; (4) ventral external arcuate
fibers from the arcuate and lateral reticular nuclei (Fig. 104).
The so-called medial part of the restiform body consists of bundles of fibers
belonging to the tractus nucleocerebellaris, which course along the medial side of
that peduncle as it turns dorsally into the cerebellum (Fig. 110). These come
from the sensory nuclei of the cranial nerves. Most of them arise from the
superior and lateral vestibular nuclei or represent the ascending branches of the
206 THE NERVOUS SYSTEM
fibers of the vestibular nerve and constitute the tractus vestibulocerebellaris .
According to Cajal (1911) the fibers of this tract are distributed to the cortex
of the cerebellum, the majority of them going to the vermis, a smaller proportion
to hemisphere. In view of the newer ideas concerning the morphology of the
cerebellum, the statements concerning the termination of all these cerebellar
afferent fibers require re-examination.
The brachium conjunctivum (Fig. 88) consists of efferent fibers from the
dentate nucleus to the red nucleus and the thalamus of the opposite side. It is
the smallest and most medial of the three peduncles. The ventral spinocere-
bellar tract enters the cerebellum in company with the brachium conjunctivum.
It ascends through the medulla oblongata and pons, curves over the brachium
conjunctivum (Fig. 110), and enters the anterior medullary velum, within which
it runs to the cerebellum (Fig. 149). Its fibers terminate in the rostral part of
the vermis and in the nucleus fastigii (Horrax, 1915). According to Edinger,
a bundle of fibers, the tectocerebellar tract, arises in the tectum of the mesencepha-
lon and descends alongside of the brachium conjunctivum to the cerebellum,
probably conveying impulses from visual centers.
According to MacNalty and Horsley (1909) and Ingvar (1918) the fibers of the ventral
spinocerebellar tract end in the lobulus centralis, culmen, and most rostral part of the declive.
The fibers of the dorsal spinocerebellar tract have the same termination and, in addition,
many of them go to the pyramis, and smaller numbers to the uvula and nodule. Practically
all of the fibers which end in the cortex, therefore, go to the anterior and posterior lobes
(Ingvar). The fact that the anterior lobe receives the majority of these fibers, which convey
proprioceptive impulses from the trunk and extremities, is a strong argument against Bolk's
conception of the anterior lobe as a co-ordination center for the musculature of the head.
HISTOLOGY OF THE CEREBELLAR CORTEX
The cerebellar cortex differs from that of the cerebral hemispheres in pos-
sessing essentially the same structure in all the lobules. This would indicate
that it functions in essentially the same way throughout, though as a result of
different fiber connections the various lobules act on different muscle groups.
A section through the cerebellum, taken at right angles to the long axis
of the folia, shows each folium to be composed of a central white lamina, covered
by a layer of gray cortex. Within the white lamina the nerve-fibers are arranged
in parallel bundles extending from the medullary center of the cerebellum into
the lobules and folia. A few at a time these bundles turn off obliquely into the
gray matter, and there is no sharp demarcation between the cortex and the sub-
jacent white lamina. The cortex presents for examination three well-defined
THE CEREBELLUM 207
zones: a superficial molecular layer, a layer of Purkinje cells, and a subjacent
granular layer.
The cells of Purkinje have large flask-shaped bodies and are arranged in an
almost continuous sheet, consisting of a single layer of cells and separating
the other two cortical zones (Fig. 150). They are more numerous at the summit
than at the base of the folium. Each has a pyriform cell body. The part
directed toward the surface of the cortex resembles the neck of a flask and from
Fig. 150. Semidiagrammatic transverse section through a folium of the cerebellum. (Golgi
method): A, Molecular layer; B, granular layer; C, white matter; a, Purkinje cell; b, basket cells;
d, pericellular baskets, surrounding the Purkinje cells and formed by the arborizations of the
axons of the basket cells; e, superficial stellate cells;/, cell of Golgi Type II; g, granules, whose
axons enter the molecular layer and bifurcate at i; h, mossy fibers; j and m, neuroglia; n, climb-
ing fibers. (Cajal.)
it spring one or two stout dendrites. These run into the molecular layer and
extend throughout its entire thickness, branching repeatedly. This branching
occurs in a plane at right angles to the long axis of the folium; and it is only in
sections, taken in this plane, that the full extent of the branching can be ob-
served. In a plane corresponding to the long axis of the folium the dendrites
occupy a more restricted area (Fig. 151). In this respect the dendritic ramifica-
tions resemble the branches of a vine on a trellis. From the larger end of the
cell, directed away from the surface of the cortex, there arises an axon which
208
THE NERVOUS SYSTEM
almost at once becomes myelinated and runs through the granular layer into the
white substance of the cerebellum. According to Clarke and Horsley (1905) and
Cajal (1911) these axons end in the central cerebellar nuclei. Near their origin
they give off collaterals, which run backward through the molecular layer to
end in connection with neighboring Purkinje cells an arrangement designed
to bring about the simultaneous discharge of a whole group of such neurons.
The granular layer, situated immediately subjacent to that which we have
just described, is characterized by the presence of great numbers of small neurons,
the granule cells. Each of these contains a relatively large nucleus, surrounded
by a small amount of cytoplasm; and from each there are given off from three
to five short dendritic branches with claw-like endings. These are synaptically
related with the terminal branches of the moss fibers, soon to be described, and
Purkinje cell"
Basket cell"
Granule cell "
"Purkinje cell
' Granule cell
Fig. 151. Diagrammatic representation of the structure of the cerebellar cortex as seen
in a section along the axis of the folium (on the right), and in a section at right angles to the axis
of the folium (on the left).
form with them small glomeruli comparable to those of the olfactory bulb (Fig.
208). Each granule cell gives origin to an unmyelinated axon, which extends
toward the surface of the folium and enters the molecular layer. Here it divides
in the manner of a T into two branches. These run parallel to the long axis of
the folium through layer after layer of the dendritic expansions of the Purkinje
cells, with which they doubtless establish synaptic relations (Fig. 151). Besides
the granules just described, this layer contains some large cells of Golgi's Type
II (Fig. 150, /). Most of these are placed near the line of Purkinje cells and
send their dendrites into the molecular layer, while their short axons resolve
themselves into plexuses of fine branches in the granular zone.
The molecular layer contains few nerve-cells and has in transverse sections
a finely punctate appearance. It is composed in large part of the dendritic
ramifications of the Purkinje cells and the branches of axons from the granule
THE CEREBELLUM
209
cells (Fig. 150). It contains a relatively small number of stellate neurons, the
more superficial of which possess short axons and belong to Golgi's Type II.
Those more deeply situated have a highly specialized form and are known as
basket cells. From each of these there arises, in addition to several stout branch-
ing dendrites, a single characteristic axon, which runs through the molecular
layer in a plane at right angles to the long axis of the folium (Fig. 151). These
axons are at first very fine, but soon become coarse and irregular, giving off
numerous collaterals which are directed away from the surface of the cortex.
These collaterals and the terminal branches of the axons run toward the Purkinje
cells, about which their terminal arborizations form basket-like networks (Fig. 29) .
Purkinje cell
Dentate nucleus ^
Brachium conjunc-
tivum
Brachium pontis
Restiform body
Climbing fibers'
Mossy fibers -'
x Basket cell
Granule cell
Fig. 152. Diagram to illustrate the probable lines of conduction through the cerebellum.
Nerve-fibers. The axons of the Purkinje cells form a considerable volume
of fibers directed away from the cortex. There are also two kinds of afferent
fibers which enter the cortex from the white center, and are known as climbing
and mossy fibers respectively. The latter are very coarse and give off numerous
branches ending within the granular layer. The terminal branches are provided
with characteristic moss-like appendages. These mossy tufts are intimately
related to the claw-like dendritic ramifications of the granule cells (Fig. 152).
The climbing fibers, somewhat finer than those of the preceding group, pass
through the molecular layer and become associated with the dendrites of the
Purkinje cells in the manner of a climbing vine. Branching repeatedly, they
14
210 THE NERVOUS SYSTEM
follow closely the dendritic ramifications of these neurons and terminate in free
varicose endings.
It would seem reasonable to suppose that the two kinds of afferent fibers,
just described, have a separate origin and functional significance. According
to Cajal (1911) it is probable that those entering the cerebellum through the
brachium pontis are distributed as climbing fibers, and those from the restiform
body as mossy fibers. The accompanying diagram represents the probable
course of impulses through the cerebellum (Fig. 152). The mossy fibers, prob-
ably derived from the restiform body, transfer their impulses to the granule
cells; and these, in turn, relay them, either directly or through the basket neu-
rons, to the Purkinje cells. The climbing fibers, which probably come from the
brachium pontis, transfer their impulses directly to the dendrites of the Purk-
inje cells. We do not known to which class the fibers of the vestibulocerebellar
tract should be assigned. The efferent path may be said to begin with the
Purkinje cells, whose axons terminate in the central cerebellar nuclei. From
these nuclei, especially the dentate, arise the fibers of the brachium conjunc-
tivum, the great efferent tract from the cerebellum. By means of the axons
of the granule cells, basket cells, and neurons of Golgi's Type II, as well as by
the collaterals from the axons of the Purkinje cells, an incoming impulse may be
diffused through the cortex.
The cerebellum probably receives fibers from all the somatic sensory centers,
but especially from those of the proprioceptive group, through which afferent
impulses are conveyed to it from the muscles, joints and tendons, and from
the semicircular canals of the ear. Its connection with the vestibular appa-
ratus is especially intimate. In fact, as already stated, it may be regarded from
the standpoint of development as a very highly specialized portion of the ves-
tibular nucleus. It is the great proprioceptive correlation center. Further-
more, it sends efferent impulses to the various somatic motor centers and plays
an important part in the coordination of muscular contraction and in the main-
tenance of muscular tone. It is the chief center for equilibration, which depends
upon the proper adjustment of the muscles in response, very largely, to the
impulses from the semicircular canals. In man and mammals it also receives
impulses from the cerebral cortex by way of the pons, which probably set the
coordinating cerebellar mechanism into activity to bring about the proper
adjustment of voluntary movements. For additional details concerning the
functions of the cerebellum the reader should consult the recent paper by
Holmes (1917).
THE CEREBELLUM
211
THE EFFERENT CEREBELLAR TRACTS
The efferent cerebellar tracts arise in the central nuclei. It is probable that
no fibers of cortical origin leave the cerebellum except, perhaps, some to Deiter's
nucleus (Clarke and Horsley, 1905).
The brachium conjunctivum, or tractus cerebellotegmentalis mesencephali,
arises for the most part at least in the dentate nucleus and terminates in the red
nucleus and thalamus of the opposite side (Fig. 153). It constitutes the chief
tract leading from the cerebellum and has been more fully described on page
159. It undergoes a complete decussation beneath the inferior colliculus in
the tegmentum of the mesencephalon. Both before and after this crossing its
Brachium conjunctivum
Thalamus
-Red nucleus
Nucleus fastigii
TV Nucleus dentatus
Fastigiobulbar tract
- Tractus cerebellotegmentalis
pontis
Lateral vestibtilar nucleus
Fastigiobulbar tract
Fig. 153. Efferent tracts which arise in the central nuclei of the cerebellum. (Modified from
Edinger.)
fibers give off branches, which descend in the reticular formation of the pons
and medulla. Some of the impulses reach the thalamus, but the others are
relayed in the red nucleus along the rubrospinal and rubroreticular tracts to
motor neurons in the brain stem and spinal cord (Fig. 115).
Other efferent tracts arise in the nucleus fastigii of the same and opposite
side, and run, probably by way of all three cerebellar peduncles, to the retic-
ular formation of the pons and medulla oblongata. One bundle of these fibers
winds around the brachium conjunctivum before descending through the pons
and medulla (Fig. 153). It is probable that other fibers descend by way of the
restiform body, and are distributed in the reticular formation of the medulla
212 THE NERVOUS SYSTEM
oblongata on the same side, or are continued as ventral external arcuate fibers
to end on the opposite side. The bundles which run from the nucleus fastigii
to the medulla oblongata may be designated as the fastigiobulbar tracts (tractus
cerebellotegmentales bulbi). These include fibers which terminate in the
lateral vestibular nucleus. It is said that some fibers belonging to this system
leave the cerebellum by way of the brachium pontis (tractus cerebellotegmentalis
pontis).
Since the dentate nucleus receives fibers from the cortex of the correspond-
ing cerebellar hemisphere, and the nucleus fastigii receives similar fibers from
the vermis, it may be inferred that the brachium conjunctivum is the chief
efferent tract for the hemisphere and that the fastigiobulbar tracts serve the
same purpose for the vermis (Strong, 1915).
CHAPTER XIV
THE DIENCEPHALON AND THE OPTIC NERVE
Development. In an earlier chapter we traced briefly the development of
the prosencephalon and showed that the cerebral hemispheres were developed
through the evagination of the lateral walls of the telencephalon (Fig. 16). It
is, however, only the alar lamina which is involved in this evagination. The
basal lamina of the telencephalon retains its primitive position and forms the
pars optica hypothalami. This part of the hypothalamus, along with the
lamina terminalis and the most rostral part of the third ventricle, constitutes
the telencephalon medium (Johnston, 1912). Through the excessive growth of
the hemisphere the diencephalon becomes covered from view (Fig. 17), and
appears to occupy a central position in the adult human brain. It is separated
from the hemisphere by the transverse cerebral fissure, which is formed by the
folding back of the hemisphere over the diencephalon. The differentiation of
the alar lamina of the diencephalon into the thalamus, epithalamus, and meta-
thalamus, and of its basal lamina into the hypothalamus was briefly traced on
page 34. The sulcus limitans, which separates these two plates in the embryo,
corresponds to the more caudal portion of the hypothalamic sulcus of the adult;
but, since the latter can be followed to the interventricular foramen, while the
former ends near the optic chiasma, the rostral ends of these two sulci are not
related. The roof plate of the prosencephalon remains thin and constitutes
the epithelial roof of the third ventricle, which along the median plane becomes
invaginated into the ventricle as the covering of a vascular network to form
the chorioid plexus.
THE THALAMUS
The thalamus is a large ovoid mass, consisting chiefly of gray matter, placed
obliquely across the rostral end of the cerebral peduncle (Figs. 154, 155). Be-
tween the two thalami a deep median cleft is formed by the third ventricle.
The rostral end is small and lies close to the median plane. It projects slightly
above the rest of the dorsal surface, forming the anterior tubercle of the thalamus,
and helps to bound the interventricular foramen (Fig. 154). The caudal ex-
tremity is larger and is separated from its fellow by a wide interval, in which the
213
214
THE NERVOUS SYSTEM
corpora quadrigemina appear. It forms a marked projection, the pulvinar,
which overhangs the medial geniculate body and the brachia of the corpora
quadrigemina (Figs. 88, 154). For purposes of description it is convenient to
recognize four thalamic surfaces, namely, dorsal, ventral, medial, and lateral.
The dorsal surface of the thalamus is free (Figs. 91, 154). It forms the
floor of the transverse fissure of the cerebrum and is separated by this fissure
from the parts of the cerebral hemisphere which overlie it, that is, from the
Free portions of columns offornix.
Head of caudate nucleus-^
Medullary strior v ""'
Third ventricle x N
Eabenular trigone ^
Pineal bodv -
Superior colliculus J
Tail of caudate nucleus-
Super, quadrigeminal brack,-
Infer, quadrigeminal brack. - '
Cerebral peduncle- '
Corpora quadrigemina-''
lateral filaments of pans--'
Anterior medullary velum- -
Lingula of cerebellum
Tela chorioidea of fourth ventricle
Corpus callosum
/Lamina of septum pellucidum
/' Columns offornix
"^ y A nterior commissure
^~ Optic recess of ventricle III
- A nterior tubercle of thalamus
, Terminal stria
^ s Tania chorioidea
^-- Habenular commissure
x'Z,awzwa affixa
^ .Superior quadrigeminal
brachium
^-' Pulvinar of thalamus
.Lateral geniculate body
~~ -Medial geniculate body
~~ Inferior colliculus
- Trochlear nerve
Brachium conjuctivum
, Lateral recess of fourth ventricle
Brachium pontis
.Peduncle of flocculus
Flocculus of cerebellum
Lateral aperture of- ventricle IV
^Chorioid plexus of ventricle IV
\Rhomboidfossa (intermediate portion)
Medial aperture of ventricle IV
Funiculus gracilis
Medulla oblongata
Fig. 154. Dorsal view of the human brain stem. (Sobotta-McMurrich.)
fornix and corpus callosum. Laterally it is bounded by a groove, which separates
it from the caudate nucleus and contains a strand of longitudinal fibers, the
stria terminalis and a vein, the vena terminalis (Figs. 154, 155). The dorsal
surface is separated from the medial by a sharp ridge, the tania thalami, which
represents the torn edge of the ependymal roof of the third ventricle. The
taeniae of the two sides meet in the stalk of the pineal body. The prominence
of this torn edge of the roof is increased by a longitudinal bundle of fibers,
THE DIENCEPHALON AND THE OPTIC NERVE
215
the stria medullaris thalami. This fascicle, together with the closely related
habenular trigone and the pineal body, belong to the epithalamus and will be
described later.
The dorsal surface of the thalamus is slightly convex and is divided by a faint
groove into two parts: a lateral area, covered by the lamina affixa and forming
a part of the floor of the lateral ventricle; and a larger medial area, which forms
the floor of the transverse fissure of the cerebrum. The oblique groove separat-
ing these two areas corresponds to the lateral border of the fornix (Figs. 154, 155).
The lamina affixa is part of the ependymal lining of the lateral ventricle superim-
Fornix
Stratum zonale
Chorioid plexus of lateral ventricle
Lamina affixa
Internal medullary
lamina
Chorioid plexus of
third ventricle
Third ventricle
Lenticular nucleus-
Internal capsule' ?
Hypothalamic
nucleus
^Transverse fissure of the cerebrum
Stria medullaris
-, Corpus callosum
Lateral ventricle
Caudate nucleus
Stria terminalis
and vena ter-
minalis
._ External medull-
ary lamina
Anterior nucleus
of thalamus
$--, ^Lateral njifleus
of thalamus
- Medial nucleus
of thalamus
Red nucleus"-
Optic tract
% Basis pedunculi
Fig. 155. Diagrammatic frontal section through the human thalamus and the structures which
immediately surround it.
Substantia nigra '
posed upon this part of the thalamus. It is not present in the sheep, where the
fornix is larger and the entire dorsal surface of the thalamus belongs to the floor
of the transverse fissure. These features are well illustrated in Figs. 179 and
180, as is also the position of the transverse fissure. This fissure intervenes be-
tween the thalamus and the cerebral hemisphere, and contains a fold of pia
mater, known as the tela chorioidea, of the third ventricle.
The medial surface of the thalamus forms the lateral wall of the third ven-
tricle (Figs. 158, 159). It is covered by the ependymal lining of that cavity.
The medial surfaces of the two thalami are closely approximated, being separated
2l6 THE NERVOUS SYSTEM
from each other by the cleft-like space of the third ventricle, and are united across
the median plane by a short bar of gray substance, the massa intermedia.
The lateral surface is hidden from view. It lies against the broad band of
fibers, known as the internal capsule, which connects the cerebral hemispheres
with the lower levels of the central nervous system. This surface is best examined
in sections through the entire cerebrum (Figs. 155-157). Many fibers stream
out of the thalamus through its lateral surface and enter the internal capsule,
through which they reach the cerebral cortex. To this important stream of
fibers the name thalamic radiation is applied.
The ventral surface of the thalamus is also covered from view and lies on the
hypothalamus, by which it is separated from the tegmentum of the mesencepha-
lon (Figs. 155, 157). Many fibers, representing such ascending tegmental paths
as the medial lemniscus, spinothalamic tract, and brachium conjunctivum, enter
the thalamus through this surface.
Structure of the Thalamus. The thalamus consists chiefly of gray matter,
within which there may be recognized a number of nuclear masses. Its dorsal
surface is covered by a thin layer of white matter, called the stratum zonale,
which in the region of the pulvinar consists in large part of fibers derived from
the optic tract. On the lateral surface of the thalamus next the internal cap-
sule there are many myelinated fibers, which constitute the external medullary
lamina (Figs. 155, 156). The medial surface is covered by a layer of central
gray matter, continuous with that which lines the cerebral aqueduct and forms
the floor of the third ventricle. This central gray matter consists of neuroglia
and of scattered nerve-fibers and cells (the nucleus paramedianus of Malone,
1910). Some of these fibers are continued through the gray matter that lines
the aqueduct and the floor of the fourth ventricle, as the dorsal longitudinal
bundle of Schutz (Fig. 112). It is probable that this portion of the thalamus
forms a center for vasomotor and visceral reflexes, since lesions in this region
are often accompanied by disturbances in the nervous control of the blood-
vessels and viscera (Edinger, 1911; Rogers, 1916). If this be true, it is probable
that the dorsal longitudinal bundle of Schutz serves to bring this thalamic
mechanism for visceral adjustments into connection with the visceral efferent
nuclei of the brain.
From the stratum zonale, which clothes its dorsal surface, there penetrates
into the thalamus a vertical plate of white matter, called the internal medullary
lamina. This subdivides the thalamus into three parts: the anterior, medial,
and lateral nuclei. At the rostral extremity of its dorsal border the internal
THE DIENCEPHALON AND THE OPTIC NERVE
217
medullary lamina bifurcates and includes between its two limbs the anterior
nucleus.
The anterior nucleus (or dorsal nucleus) of the thalamus is located in the
dorsal part of the rostral extremity of the thalamus and penetrates like a wedge
between the medial and lateral nuclei. It protrudes somewhat above the
general level of the dorsal surface, forming the anterior tubercle of the thalamus.
It receives a large bundle of fibers from the mammillary body, the mamillotha-
lamic tract or bundle of Vicq d'Azyr (Figs. 156, 204, 205), and sends fibers to the
caudate nucleus of the corpus striatum (Fig. 196).
TatHia tecia Stria* Lancisii
Subitaut'ui nigra
catidatta
Nticleta anterior tkalami
KucUm lakralis tkalami
Nuclrmt miJiafa tkalami
lint thatamiaa
nsa lenticularis
Trattiu oftlcia
Pa feduuaiU
Fig. 156. Frontal section through the mammillary body, thalamus, and adjacent structures.
Weigert method. (Villiger-Piersol.)
The medial nucleus of the thalamus is situated between the central gray
matter of the third ventricle and the internal medullary lamina, which separates
it from the lateral nucleus except in the caudal part, where the line of separation
between the two is not distinct. It is said to receive fibers from the olfactory
centers and to send fibers to the caudate nucleus and the subthalamus.
The lateral nucleus of the thalamus is by far the largest of the three. It
extends farther caudad than the medial nucleus and includes all of the pulvinar.
Through the external medullary lamina and the internal capsule it sends fibers
to the cerebral cortex in the thalamic radiation and receives corticothalamic
2l8
THE NERVOUS SYSTEM
fibers in return. Especially in its ventral subdivision it receives all of the as-
cending sensory tracts from the tegmentum of the mesencephalon, as well as
fibers from the brachium conjunct! vum and red nucleus. It is much more richly
supplied throughout with myelinated fibers than are the other nuclei of the thala-
mus.
The lateral nucleus is subdivided into a dorsal portion, the lateral nucleus
proper, and a ventral part, better known as the ventral nucleus of the thalamus.
Within the latter are two well-defined nuclear masses. The more medial of
Tunis umlclrcula
Corfus gtniatlatitm lattrale
Gyrus dentalus
Fig. 157. Frontal section through the human pons, basis pedunculi, thalamus and adjacent
structures. Weigert method. (Villiger-Piersol.)
the two is known as the nucleus centralis (nucleus globosus or centrum media-
num) and is surrounded by a well-defined capsule of myelinated fibers (Fig. 157).
Ventrolateral to this is the well-defined nucleus arcuatus, which because of its
shape is also called the nucleus semilunaris. The pulvinar is a very large mass
which forms the most caudal part of the thalamus and is usually considered as
a part of the lateral nucleus.
Function. The medial and anterior thalamic nuclei are closely associated in
function and from a phylogenetic point of view represent the older part of the
thalamus. They serve as centers for the more primitive thalamic correlations
THE DIENCEPHALON AND THE OPTIC NERVE 219
such as occur in lower vertebrates that lack the cerebral cortex (Herrick, 1917)
Both receive fibers from the olfactory centers and both send fibers to the corpus
striatum, but none to the cerebral cortex (Sachs, 1909). There is some evidence
of a clinical nature to show that the activity of these centers may be accompanied
by a crude form of consciousness (Head and Holmes, 1911; Head, 1918). Pa-
tients in whom the paths from the thalamus to the cortex have been interrupted
are aware of many sensations, but cannot discriminate among them. The
thalamus seems to be the chief center for the perception of pain and the affec-
tive qualities of other sensations, and in this respect it plays an important
role in consciousness independently of the cerebral cortex.
The more lateral group of centers, which includes the lateral nucleus of the
thalamus, the pulvinar, and the geniculate bodies, is of more recent origin and
has been called the neothalamus. They serve as relay stations on the somatic
sensory paths to the cerebral cortex. The medial lemniscus and spinothalamic
tracts terminate in the ventral subdivision of the lateral nucleus. In the pul-
vinar and lateral geniculate body terminate fibers from the optic tracts, while
the lateral lemniscus ends in the medial geniculate body. From these nuclei
sensory fibers of the third order run to the cerebral cortex. The lateral nucleus,
exclusive of the pulvinar, is therefore a relay station on the paths of cutaneous
and deep sensibility, and it is connected with the parietal and frontal cortex
through the thalamic radiation. The pulvinar and lateral geniculate body are
stations on the optic pathway, and the medial geniculate body on that for hearing.
The thalamic radiation can best be considered in detail after we have ac-
quired some familiarity with the structure of the cerebral hemisphere (p. 263).
The fiber tract connections, established by the various nuclear masses composing the
thalamus, among themselves and with other parts of the brain, are not as yet well known.
This is particularly true of the descending tracts. It is known that from the region of the
thalamus a large bundle, the thalamo-olivary tract, descends to the inferior olivary nucleus.
Some authors also describe a thalamospinal tract which arises in the thalamus and is closely
associated with the rubrospinal tract.
It is fairly well established that each of the ascending sensory tracts of the tegmentum
has its own particular field of distribution within the ventral nucleus of the thalamus; and it
is, therefore, probable that there are corresponding functional differences in the various
subdivisions of this nucleus. Beginning at the lateral side and passing medialward, the
terminals of these various tracts are as follows: The spinothalamic tract ends in the most
lateral part of the ventral nucleus. Next comes the field, within which terminate the fibers
of the central tract of the trigeminal nerve, and which includes the nucleus arcuatus and
nucleus centralis. The medial lemniscus ends in the most medial part of the inferior nucleus,
including the nucleus centralis. This corresponds to the relative position which these tracts
occupy in the tegmentum of the mesencephalon, where the spinothalamic tract is the most
lateral of the three.
22O THE NERVOUS SYSTEM
THE METATHALAMUS
The metathalamus is composed of two small protuberances, the geniculate
bodies, which, having been displaced by the excessive development of the
thalamus, are situated upon the dorsolateral surface of the rostral end of the
mesencaphalon (Figs. 87-89, 154, 161). The lateral geniculate body is an oval
swelling in the course of the optic tract. Its connections will be more fully
considered in connection with the discussion of the course of the visual impulses.
The medial geniculate body is overhung by the pulvinar, from which it is separated
by a deep sulcus. It receives fibers by way of the inferior quadrigeminal bra-
chium from the lateral lemniscus, which we have learned to know as the central
auditory path from the cochlear nuclei. From it fibers run to the auditory
area of the cerebral cortex (the thalamotemporal or acoustic radiation).
THE EPITHALAMUS
The epithalamus includes the pineal body, stria medullaris, and habenular
trigone. The latter is a small triangular depressed area located on the dorso-
medial aspect of the thalamus rostral to the pineal body (Fig. 158) . In the sheep,
as in most other mammals, it is much larger than in man and bulges both dor-
sally and medially beyond the surface of the thalamus (Figs. 91, 159). It marks
the position of a nuclear mass, called the habenular ganglion, which receives fibers
from the stria medullaris, a fascicle which runs along the border between the
dorsal and medial surfaces of the thalamus subjacent to the taenia thalami
(Figs. 154, 155). The stria medullaris takes origin from the anterior perforated
substance and other olfactory centers on the basal surface of the cerebral hemi-
sphere and, partially encircling the thalamus, reaches the habenular ganglion,
in which it ends. (See p. 281.) Not all of the fibers terminate on the same
side; some cross to the ganglion of the opposite side, forming a transverse bundle
of myelinated fibers which joins the caudal end of the two ganglia together and
is known as the habenular commissure. From the cells in this ganglion arises
a bundle of fibers, known as the fasciculus retroflexus of Meynert or the tractus
habenulopeduncularis. This bundle of fibers is directed ventralward and at
the same time caudally along the medial side of the red nucleus toward the
base of the brain, where it crosses to the opposite side and ends in the inter-
peduncular ganglion (Fig. 189). The stria medullaris, habenular ganglion,
and fasciculus retroflexus are all parts of an arc for olfactory reflexes, as indi-
cated in Fig. 211. According to Edinger (1911) the cells, from which the stria
medullaris arises, are intimately related to a bundle of ascending fibers from
THE DIENCEPHALON AND THE OPTIC NERVE
221
the sensory nuclei of the trigeminal nerve. If this be true, the mechanism in
question may receive afferent impulses from the nose, mouth, and tongue and
be concerned with feeding reflexes.
The pineal body is a small mass, shaped like a fir cone, which rests upon the
mesencephalon in the interval between the two thalami. Its base is attached
by a short stalk to the habenular and posterior commissures, and into the stalk
there extends the small pineal recess of the third ventricle. The pineal body is
a rudimentary structure and is not composed of nervous elements. In some
Hypothalmic sulcus
Habenula
Habenular commissure^ \
Suprapineal recess i >
\ \ \
Posterior commissure \ \ \
Pineal body \ \ \
Splenium of corpus callosum N \ \ \ *. '
Lamina quadrigemina \ \ \
\ N ^/ X v >
Cerebral aqueduct N N ^ v x s A \ \ \^-^
s N v v'-^rr
Anterior medullary velum,
Fourth ventricle N
5w/>. z;erw. of cerebellum x
Fissura prima N_J
Inferior vermis
of cerebellum
Epithelial roof and chori-
oid plexus of fourth'''
ventricle
, Body of fornix
Chorioid plexus of third ventricle
Massa intermedia
Epithelial roof of third ventricle
I Lamina commissure hippocampi
1 t Corpus collosum
Genu of corpus
' callosum
Septum pelluci-
J^'^ dum
^"~~ Ros.ofcor. callosum
~ - Lamina rostralis
" Columna fornicis
^-~~^ Interventr icular foramen
^^.^ Anterior commissure
*-^~ Lamina terminalis
^-C^Optic recess
^*- ^Optic chiasma
~ -^ Infundibulum
Hypophysis
Mammillary body
Oculomotor nerve
\ *Subthalamus
Tegmentum of mesencephalon
Pans
Medulla
'Central canal
Fig. 158. Median sagittal section through the human brain stem.
vertebrates, certain lizards for example, it is more highly developed, resembles
in structure an invertebrate eye, and lies close to the dorsal surface of the head.
The posterior commissure is a large bundle of fibers which crosses the median
plane dorsal to the point where the cerebral aqueduct opens into the third
ventricle (Figs. 154, 156). The source and termination of the fibers which
constitute the bundle are still obscure.
222 THE NERVOUS SYSTEM
THE HYPOTHALAMUS
The hypothalamus consists of three parts: (1) the pars optica hypothalami,
which belongs to the telencephalon, (2) the pars mamillaris hypothalami, and
(3) the subthalamus.
The pars mamillaris hypothalami includes the corpora mamillaria, tuber
cinereum, infundibulum, and hypophysis. The mammillary bodies are a pair of
small spheric masses of gray matter, situated close together in the interpedun-
cular space rostral to the posterior perforated substance (Figs. 86, 158, 159).
Each is enclosed in a white capsule and projects as a rounded white eminence
at the base of the brain (Fig. 156). In the sheep's brain the two are fused to-
gether into a single eminence (Fig. 83). Each mammillary body is composed
of two nuclear masses: a large medial group of small cells and a smaller lateral
collection of large cells. The white capsule is formed by fibers from the hippo-
campus, which sweep in a broad curve around the thalamus, forming a bundle
known as thefornix (Figs. 204, 205). This descends in front of the interventric-
ular foramen and reaches the mammillary body, within, which a large part of
these fibers end. From the dorsal aspect of the medial nucleus springs a stout
fascicle, which runs dorsally, to end in the anterior nucleus of the thalamus, and
is known as the mammillothalamic tract or bundle of Vicq d'Azyr (Figs. 156, 204,
205). A short distance from the mammillary body there branches off from this
tract another, the mammillotegmental tract of Gudden, which runs caudally in
the tegmentum of the mesencephalon and probably ends in the dorsal tegmental
ganglion. The lateral nuclear mass is also connected with the tegmentum by
way of the peduncle of the mammillary body (Fig. 211).
The tuber cinereum, as seen -from the ventral surface of the brain (Figs.
83, 86), is a slightly elevated gray area rostral to the mammillary bodies. It is
one of the olfactory centers. To it there is attached the funnel-shaped stalk
of the hypophysis, known as the infundibulum. The hypophysis is a small
gland of internal secretion, which is not composed of nervous tissue and which
interests us here only because its posterior portion is developed as an outpock-
eting of the ventral wall of the diencephalon, to which it remains attached by
the infundibulum. A detailed account of this structure may be found in the
papers by Tilney (1911 and 1913) listed in the Bibliography at the end of this
volume.
The subthalamus is situated between the thalamus and the tegmentum of
the mesencephalon and forms a zone of transition between these two struc-
tures (Figs. 156, 157). The long sensory tracts of the tegmentum run through
THE DIENCEPHALON AND THE OPTIC NERVE
223
it on their way to the thalamus. The red nucleus and the substantia nigra
project upward into it from the mesencephalon. An additional mass of gray
matter is found in this region lateral to the red nucleus and ventral to the thala-
mus. It is known as the hypothalamic nucleus and has the shape of a biconvex
lens. Its function and fiber connections are not well understood; but it is prob-
ably a motor coordination center receiving fibers from the thalamus, corpus
striatum, and pyramidal tract, and sending fibers downward in the cerebral
peduncle.
THE THIRD VENTRICLE
Since the third ventricle is chiefly surrounded by structures belonging to the
diencephalon, it will be convenient to consider it at this point and to give at
Interventricular foramen Body of corpus callosum
Anterior commissure | ' \ Body of fornix
Septum pellucidum^ \ \ \
Rostral lamina \ \ \
Rostrum of corpus callosum. > \ \ '<
Genu of corpus callosum , ', \ ', \ <
Hippocampal com. Roofs of third ventricle or tela chorioidea
Stria med.
Habenular
Trigone
/' Haben. com.
,' Splenium
',' Pineal
! i body /
Suprapineal recess
.Superior colliculus
/Primary fissure
/White center of vermis
Olfactory bulb , .
Medial olfactory gyms' /,
Anterior perf. substance' /
Lamina terminalis /
Diagonal band
Central canal
\ Medulla
Medial aperture of
\ fourth ventricle '
\ \Tela chorioidea
' Fourth ventricle
'Anterior medullary
velum
Fig. 159. Medial sagittal section of the sheep's brain.
the same time an account of the parts of the telencephalon which help to form
its walls. These include the lamina terminalis, anterior commissure, and the
optic chiasma (Figs. 158, 159). The latter, formed by the decusssation of the
fibers of the optic nerve, projects as a transverse ridge in the floor of the ven-
tricle. The lamina terminalis is a thin plate joining the two hemispheres, which
stretches from the optic chiasma in a dorsal direction to the anterior commis-
sure. Here it becomes continuous with the thin edge of the rostrum of the
corpus callosum, known as the rostral lamina. As indicated on page 26, the
224 THE NERVOUS SYSTEM
lamina terminalis is to be regarded as forming the rostral end of the brain;
and the part of the third ventricle, which lies behind it and dorsal to the optic
chiasma, belongs to the telencephalon. The anterior commissure is a bundle of
fibers which crosses the median plane in the lamina terminalis and serves to
connect certain parts of the two cerebral hemispheres, which are associated with
the olfactory nerves. The anterior commissure and the lamina terminalis form
the rostral boundary of the third ventricle, and between the latter and the optic
chiasma is a diverticulum, known as the optic recess.
The third ventricle is a narrow vertical cleft, the lateral walls of which are
formed for the greater part by the medial surfaces of the two thalami. Ventral
to the massa intermedia is seen a groove known as the hypothalamic sulcus, which
if followed rostrally leads to the interventricular foramen, while in the other
direction it can be traced to the cerebral aqueduct. Below this groove the
lateral wall and floor of the ventricle are formed by the hypothalamus.
In the floor of the ventricle there may be enumerated the following structures,
beginning at the rostral end: the optic chiasma, infundibulum, tuber cinereum,
mammillary bodies, and the subthalamus.
The roof of the third ventricle is formed by the thin layer of ependyma, which
is stretched between the striae medullares thalami of the two sides, and whose
torn edge, in the dissected specimen, is represented by the tania thalami (Figs.
85, 155, 159). Upon the outer surface of this ependymal roof is a fold of pia
mater in the transverse fissure. This is known as the tela chorioidea; and from
it delicate vascular folds are invaginated into the ventricle, carrying a layer of
ependyma before them by which they are, in reality, excluded from the cavity.
These folds are the chorioid plexuses. There are two of them extending side by
side from the interventricular foramina to the caudal extremity of the roof.
Here they extend into an evagination of the roof above the pineal body, known
as the suprapineal recess.
There are three openings into the third ventricle. The aqueduct of the cere-
brum opens into it at the caudal end; while at the opposite extremity it com-
municates with the lateral ventricles through the two interventricular foramina.
THE VISUAL APPARATUS
Development of the Retina and Optic Nerve. There is but one pair of
nerves associated with the diencephalon, and these, the optic nerves, are not
true nerves, but fiber tracts joining the retinae with the brain. It will be re-
membered that the retina develops as an pagination of the lateral wall of the
prosencephalon in the form of a vesicle whose cavity is continuous with that of
THE DIENCEPHALON AND THE OPTIC NERVE
225
the forebrain. By a folding of its walls in the reverse direction, i. e., by invag-
ination, the optic vesicle becomes transformed into the optic cup (Fig. 15) ; and
the cavity of the vesicle becomes reduced to a mere slit between the two layers
forming the wall of the cup. The inner of these two layers develops into the
nervous portion of the retina; and nerve-fibers arising in it grow back to the brain
along the course of the optic stalk, which still connects the optic cup with the
forebrain. This mode of development serves to explain why the structure of
the retina resembles that of the brain more than it does that of other sense
organs, and why the optic nerve-fibers, like those of the fiber tracts of the cen-
tral nervous system, are devoid of neurilemma sheaths. These fibers take origin
from the ganglion cells of the retina, the structure of which must be briefly con-
sidered at this point.
Optic nerve
Optic chiasma
GanglioniciStratum oplicum ^-
neurons \ Ganglionic layer "
( Inner molecular layer
Bipolar } Inner nuclear layer
neurons I
^ Outer molecular layer
Rod and f uter nuckar layer
cone < Ex. limiting membrane
neurons \Layerofrodsandcones
Optic tract
Lateral geniculate body
Medial geniculate body
i * Pulvinar
Superior colliculus
Fig. 160. Schematic representation of the retina and the connections established by the optic
nerve-fibers.
The retina presents for consideration three layers of superimposed nervous
elements: (1) the visual cells, (2) the bipolar cells, and (3) the ganglion cells
(Fig. 160). These, with some horizontally arranged association neurons and
supporting elements, form the nervous portion of the retina and are derived
from the inner layer of the optic cup. The pigmented stratum of the retina is
derived from the outer layer of the cup.
The visual cells are bipolar elements, whose perikarya are located in the
outer nuclear layer (Fig. 160). Each presents an external process in the form of
a rod or cone, so differentiated as to respond to photic stimulation and thus to
serve as a visual receptor. The other process terminates in the outer molecular
layer in relation to processes from the bipolar cells. These latter elements have
their perikarya in the inner nuclear layer and branches in the inner and outer
molecular layers. The ganglion cells send their dendrites into the inner molec-
ular layer, where they are related to the inner branches of the bipolar cells;
15
226
THE NERVOUS SYSTEM
while the axons form the innermost stratum of the retina, the stratum opticum,
through which they enter the optic nerve. It will be apparent from Fig. 160
that the visual cells are the receptors and neurons of the first order in the optic
path. The impulses are transmitted through the bipolar cells to the ganglion
cells, whose axons, in turn, carry them by way of the optic nerves to the supe-
rior colliculus, lateral geniculate body, and pulvinar of the thalmus. In the same
figure it may be seen that the nerve also contains some efferent fibers which
terminate in the retina (Arey, 1916).
The Optic Chiasma and Optic Tracts. The optic nerve emerges from the
bulbus oculi at the nasal side of the posterior pole and, after entering the cranium
through the optic foramen, unites with its fellow of the opposite side to form the
Pulvinar of thalamus Aqueduct of cerebrum Red nucleus
Medial geniculate body,
' \ .^fr ' *'' *j5^L/* > \^ ubstantia ni & ra
Lateral geniculate body ^F ^Mlr ""* ''' iBPL X^lfc '' 'Base of peduncle
\
Cerebral peduncle ''
Optic tract -'xj
4Q
Posterior perforated substance ''
Mammillary body
* Anterior perforated substance
Tuber cinereum
Optic nerve
'Olfactory trigone
\
| Ynfundibulum
Optic chiasma
Fig. 161. The connections and relations of the optic tracts. The mesencephalon has been cut
across and the specimen is viewed from below. (Sobotta-McMurrich).
optic chiasma, in which a partial decussation of the fibers takes place (Figs.
161, 162). Beyond the decussation fibers from both retinae are continued in
each of the optic tracts. In the chiasma the fibers from the two optic nerves
are so distributed that each tract receives the fibers from the lateral half of the
retina of its own side and those from the medial half of the opposite retina.
The optic tracts partially encircle the ends of the cerebral peduncles. Each
tract divides into a medial and a lateral root, of which the former goes to the
medial geniculate body and does not consist of optic nerve-fibers. The lateral
root is much larger and runs to the lateral geniculate body and pulvinar of the
thalamus and to the superior colliculus of the corpora quadrigemina. In addi-
tion to the optic fibers each tract contains a bundle of fibers, known as the com-
THE DIENCEPHALON AND THE OPTIC NERVE
227
mis sure of Gudden, which crosses the median plane in the posterior part of the
optic chiasma and, for the most part at least, connects the medial geniculate
bodies of the two sides. These are the fibers which form the medial root of the
optic tract.
The Optic Radiation. The superior colliculus is a reflex center, and the fibers
of the optic nerve, which terminate in it, subserve optic reflexes. On the other
hand, the visual impulses, brought to the external geniculate body and the pul-
Superior oblique muscle
Retina
Optic nerve
Optic chiasma
Commissure of Gudden
Trochlear nerve
O.ptic trad
Thalamus
Medial geniculate body
Lateral genictilate body
"~ -Superior colliculus
~~- Inferior colliculus
^^Nucleus oftrochlear nerve
radiation
Cuneus
Occipital pole
Fig. 162. Schematic representation of the optic pathways. The index line to the commissure of
Gudden does not reach that structure.
vinar of the thalamus, are relayed to the cerebral cortex and give rise to visual
sensations. These two parts of the diencephalon are connected with the cere-
bral cortex on both sides of the calcarine fissure by projection fibers, which
form a conspicuous bundle that sweeps backward through the retrolenticular
portion of the internal capsule into the occipital lobe. It is known as the optic
radiation (Fig. 162). In addition to corticipetal fibers arising in the pulvinar
and lateral geniculate body, the optic radiation contains corticifugal fibers
228 THE NERVOUS SYSTEM
arising in the cortex and terminating in the pulvinar, lateral geniculate body,
and superior colliculus of the corpora quadrigemina.
The significance of the partial decussation of the nerves is made clear by
Figs. 162 and 163. The properties of the refracting media of the eyes are such
that images of objects to the left of the axis of vision are produced on the nasal
side of the left eye and the temporal side of the right eye. And, due to the man-
ner of decussation of the optic nerve-fibers, impulses from both these sources
reach the visual area of the right cortex. In the same way the visual cortex
of the left side receives impressions from objects to the right of the axis of vision.
That is to say, the sensory representation of the outer world in the cerebral
cortex is contralateral in the case of sight just as it is in the case of cutaneous
Fig. 163. Diagram to show why a destruction of one optic tract causes blindness in both eyes for
the opposite lateral half of the field of vision.
and auditory sensations. Furthermore, it will be evident that, while destruc-
tion of one optic nerve causes total blindness in the corresponding eye, destruc-
tion of one optic tract, its thalamic connections, their optic radiations, or the
visual cortex in which these radiations terminate, will produce blindness in both
eyes for the opposite lateral half of the field of vision. This condition is known
as hemianopsia, and is produced by a lesion in the optic pathway anywhere be-
hind the chiasma.
CHAPTER XV
THE EXTERNAL CONFIGURATION OF THE CEREBRAL
HEMISPHERES
Development. The cerebral hemispheres are formed by the evagination of
the alar laminae of the telencephalon, the rest of which remains as the boundary
of the rostral part of the third ventricle, and is known as the telencephalon
medium. The cavities of the evaginated portions are known as the lateral ven-
tricles and communicate with the third ventricle by way of the interventricu-
lar foramina (Figs. 15-17). Each of the cerebral hemispheres consists of two
ventrally placed portions, the rhinencephalon or olfactory lobe and corpus stria-
turn, and a third part, more extensive than the others, the pallium or primitive
cerebral cortex. The pallium expands more rapidly than the other parts, both
rostrally and caudally, and comes to overlie the diencephalon, from which it is
separated by the transverse fissure (Fig. 17). The fold of pia mater which is
inclosed within this fissure is known as the tela chorioidea; and from it a vascular
plexus grows into the lateral ventricle through the thin portion of the medial
wall of the hemisphere, where this is attached to the diencephalon. This forms
the chorioid plexus of the lateral ventricle and carries before it an epithelial cover-
ing from the ependymal lining, by which it is, in reality, excluded from the
ventricular cavity. This invagination of the medial wall of the hemisphere
produces the chorioid fissure. Ventrally the thickened part of the hemisphere,
known as the corpus striatum, remains in uninterrupted continuity with the
thalamus.
At first the cerebral hemisphere has a relatively large cavity and thin walls.
As the pallium and ventricle enlarge they become bent around the thalamus
and corpus striatum (Fig. 17). The hemisphere becomes bean shaped and
the cavity curved. It expands rostrally to form the frontal lobe, caudally to
form the occipital lobe, and ventrolaterally to form the temporal lobe (Fig. 164).
Into each of these there is carried a prolongation of the lateral ventricle forming
respectively the anterior, posterior, and inferior horns. Between the temporal
and frontal lobes a deep fossa appears which is the forerunner of the lateral
fissure. At the bottom of this fossa is the insula, a portion of the cortex which
229
230
THE NERVOUS SYSTEM
overlies the corpus striatum and develops more slowly than the surrounding areas
(labelled lateral fissure. Fig. 164). Folds from the surrounding cortex close in
over the insula, burying it from sight in the adult brain. These folds are known
as the opercula, and the deep cleft which separates them as the lateral fissure.
Development of the Cerebral Cortex. At first the pallium, like other parts
of the neural tube, consists of three primitive zones: the ependymal, mantle,
and marginal layers. But during the third month neuroblasts migrate outward
from the ependymal and mantle layers into the marginal zone and there give
rise to a superficial layer of gray matter the cerebral cortex. Nerve-fibers
from these neuroblasts and others growing into the hemisphere from the thala-
Suicus postcenlralls
Sulcus centralis
Lobus
parietalis
superior
Supra-
Occipital
pole
Inferior
frontal
sulcus
Ascend-
ing
ramus
Lateral
fissure
(Syhii)
Temporal
lobe
Superior temporal gyms Middle temporal gyrus
Fig. 164. Lateral view of the right cerebral hemisphere from a seven months' fetus. (Kollmann.)
mus accumulate on the deep surface of the developing cortex and form the
white medullary substance of the hemisphere. As the brain increases in size
the area of the cortex expands out of proportion to the increase in volume of
the white medullary layer upon which it rests, and is thrown into folds or gyri
separated by fissures or sulci. All the larger mammalian brains present well-
developed gyri, while the smaller brains are smooth; and it would thus appear
that the size of the brain is an important factor in determining the* amount of
folding that occurs in the cortex.
As we shall learn, the cortex does not differentiate in exactly the same man-
ner throughout, but may be subdivided into structurally and functionally dis-
THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 231
tinct areas. The sulci develop in more or less definite relation to these areas,
the great majority making their appearance along the boundary lines between
them. These are known as terminal sulci, of which the rhinal fissure and central
sulcus are examples. Sometimes the folding occurs entirely within such an
area, i. e., along its axis, producing what is known as an axial sulcus. But
there are still others in which the relation to these functional areas is not so evi-
dent. The arrangement of the fissures and sulci in a seven month fetus is shown
in Fig. 164.
The Development of the Septum and Commissures. The two hemispheres
are connected by the lamina terminalis, which serves as a bridge for fibers which
cross from one hemisphere to the other. These form three important bundles:
Fig. 165. Schematic representation of the development of the septum pellucidum and
telencephalic commissures: A. C., Anterior commissure; C. C., corpus callosum; C. F., columna
fornicus; C. S. P., cavum septi pellucidi; F., fornix; H. C., hippocampal commissure; /. F.,
interventricular foramen; Fis., chorioid fissure; L. T., lamina terminalis. (Based on drawings of
models of the telencephalon of a four months' fetus (.4) and of a five months' fetus (B) by Streeter.)
the anterior commissure, the hippocampal commissure, and the corpus callosum.
The two former connect the olfactory portions of the hemispheres, while the
latter is the great commissure of the non-olfactory cortex or neopallium. Every-
one admits that the anterior commissure develops in the lamina terminalis
(Fig. 165); and the corpus callosum and hippocampal commissures are said to
form in its dorsal part (Streeter, 1912). According to this account the lamina
terminalis becomes stretched by the great development of the corpus callosum
and appropriates part of the paraterminal body. This is the portion of the
rhinencephalon that lies immediately rostral to the lamina terminalis in the
medial wall of each hemisphere. Eventually the lamina terminalis presents a
large cut surface in the median sagittal section and includes the commissures
232 THE NERVOUS SYSTEM
as well as the septum pellucidum. The portion of the lamina terminalis which
enters into the formation of the septum becomes hollow as a result of the stretch-
ing to which it is subjected, and the resulting cavity is known as the cavum septi
pellucidi.
The cerebral hemispheres are incompletely separated from each other by
the longitudinal fissure of the cerebrum, at the bottom of which lies a broad band
of commissural fibers, the corpus callosum, which forms the chief bond of union
between them. Each hemisphere has three surfaces: a convex dorsolateral
surface (Fig. 166), a median surface flattened against the opposite hemisphere
(Fig. 170), and a very irregular ventral or basal surface. A dorsal border sepa-
rates the dorsolateral from the medial surface; and a lateral border marks the
transition between the dorsolateral and basal surfaces. One may recognize
also frontal, occipital, and temporal poles (Fig. 166). The long axis of the hemi-
sphere extends between the frontal and occipital poles, and in man is placed
almost at right angles to the long axis of the body (Fig. 33) ; while in other mam-
mals it corresponds more nearly to the body axis. On this account it will be
convenient in the description of the human cerebral hemisphere to take the
occiput as a point of reference and use the term "posterior" in place of "caudal."
Otherwise our directive terms remain the same rostral, dorsal, and ventral
except that for the term "ventral" we shall often use the word "basal."
The cerebral cortex is a layer of gray matter spread over the surface of the
hemisphere; and its area is greatly increased by the occurrence of folds or gyri
separated by deep sulci. That part of the cortex which belongs to the rhinen-
cephalon and is phylogenetically the oldest is designated as the archipallium.
It is separated from the newer and in mammals much larger neopallium or non-
olfactory cortex by the rhinal fissure (Figs. 83, 171).
The Neopallium. The development of the neopallium is so much greater
in man than in the sheep, and the arrangement of the gyri and sulci is so dif-
ferent in the two forms that but little can be learned by a cursory comparison of
these structures in the two brains. We shall, accordingly, confine our atten-
tion almost exclusively to the arrangement of the neopallium in man.
THE DORSOLATERAL SURFACE OF THE HEMISPHERE
By means of some of the more important sulci the cortex is marked off into
well-defined areas, known as the frontal, parietal, temporal, and occipital lobes
(Fig. 167). To these should be added a lobe buried at the bottom of the lateral
fissure and known as the insula (Fig. 169). In the delimitation of these lobes
THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
233
the lateral fissure and the central sulcus play a prominent part. Some of the
more important sulci are designated as fissures. This usage is regulated by
custom, but it may be said that a number of the fissures are invaginations of
the entire thickness of the wall of the hemisphere and produce corresponding
elevations projecting into the lateral ventricle.
The lateral cerebral fissure, or fissure of Sylvius, begins on the basal sur-
face of the brain as a deep cleft lateral to the anterior perforated substance
(Fig. 172). From this point it extends lateralward between the temporal and
frontal lobes to the lateral aspect of the brain, where it divides into three branches
(Figs. 166, 167). The anterior horizontal ramus of the lateral fissure runs ros-
Operculum
Opercular portion of inferior
frontal gyr,
Superior frontal
gyrus
Middle frontal
gyrus
Frontal pole
Triangular portion **
of inf. front, gyrus ..*
Lateral cerebral fissure^''
Temporal pole'
Superior temporal gyrus-'
Superior temporal sulcus''
Middle temporal gyrus
Middle temporal sulcus
Inferior temporal gyrus
Precentral sulcus
f Anterior central gyrus
Central sulcus
' Posterior central gyrus
'' Inter parietal sulcus
Supramarginal gyrus
Interparietal sulcus
Angular gyrus
Superior parietal
- lobule
'-^-Inferior parietal
lobule
-'' Parieto-occipital
fissure
^Lateral occipital
gyri
Occipital pole
Transverse occipital sulcus
\ Superior temporal sulcus
Posterior limb of lateral cerebral fissure
Fig. 166. Lateral view of the human cerebral hemisphere. (Sobotta-McMurrich.)
trally and the anterior ascending ramus dorsally into the frontal lobe. The
posterior ramus of the lateral fissure is much longer, and runs obliquely toward the
occiput and at the same time somewhat dorsally. The terminal part turns
dorsally into the parietal lobe. This fissure is, in reality, a deep fossa, at the
bottom of which lies the insula. It separates the frontal and parietal lobes
which lie dorsal to it from the temporal lobe.
The central sulcus or fissure of Rolando runs obliquely across the dorsolateral
surface of the hemisphere, separating the frontal from the parietal lobe (Figs.
166, 167). It begins on the medial surface of the hemisphere a little behind the
middle of the dorsal border and extends in a sinuous course rostrally and toward
234
THE NERVOUS SYSTEM
the base, nearly reaching the posterior ramus of the lateral fissure. It makes
an angle of about 70 degrees with the dorsal border. It is customary to recog-
nize two knee-like bends in this sulcus; one located at the junction of the dorsal
and middle thirds with concavity forward, and the other at the junction of the
middle and basal thirds with concavity backward. If the margins of the sulcus
are pressed apart a deep annectant gyrus may often be seen extending across
it, by which the continuity of the sulcus is to some extent interrupted. This is
explained by the fact that the sulcus usually develops in two pieces, which be-
come united as the depth of the sulcus increases.
Lobes, The frontal lobe lies dorsal to the lateral cerebral fissure and rostral
to the central sulcus (Fig. 167). The remainder of the dorsolateral surface is
subdivided rather arbitrarily into the parietal, occipital, and temporal lobes.
Frontal pole
Lateral ( Ant. hor. ram.
cerebral I Ant. ascend, ram. '
fissure { Post. ram. "
Temporal pole'
-Parietal lobe
,'- Temporal lobe
-- Parieto-occipital fissure
r - Occipital lobe
Preoccipital notch
''Occipital pole
Fig. 167. Diagram of the lobes on the lateral aspect of the human cerebral hemisphere.
The rostral border of the occipital lobe is usually placed at a line joining the end
of the parieto-occipital fissure with the preoccipital notch. The latter is a
slight indentation on the lateral border of the hemisphere about 4 cm. rostral
to the occipital pole; while the parieto-occipital fissure is a deep cleft on the
median surface (Fig. 170), which cuts through the dorsal border about midway
between the occipital pole and the central sulcus, but a little nearer the former.
The parietal lobe is situated between the central sulcus and the imaginary line
joining the parieto-occipital fissure with the preoccipital notch. It lies dorsal
to the lateral fissure and an imaginary line connecting that fissure with the
middle of the preceding line. The remainder of the dorsolateral surface belongs
to the temporal lobe.
The Frontal Lobe. The rostral part of the hemisphere is formed by the
THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
235
frontal lobe. Within it one may identify three chief sulci, which are, however,
subject to considerable variation. The precentral sulcus is more or less parallel
with the central sulcus and is often subdivided into two parts, the superior and
inferior precentral sulci (Fig. 168). The superior frontal sulcus usually begins
in the superior precentral sulcus and runs rostrally, following in a general way
the curvature of the dorsal border of the hemisphere which it gradually ap-
proaches. The inferior frontal sulcus usually begins in the inferior precentral
sulcus and extends rostrally, arching at the same time toward the base of the
hemisphere.
Between the precentral and central sulci lies the anterior central gyrus in
which is found the motor area of the cerebral cortex. The remainder of this
Anterior central gyrus
Superior precentral sulcus \
Superior frontal gyrus
Superior frontal sulcus ----- .^^
Middle frontal gyrus-.
Middle frontal sulcus -/^
Inferior frontal sulcus V_ ~,~*
Inferior precentral sulcus I
Inf. (Parsopercularis -1 ~_~~~
front. \ Pars triang. -4
gynts\ Pars orbitalis-\ _
Lateral ( Ant. nor. ram.
cerebral] Ant. ascend, ram.'' /
fsstire ( Post, ram..'''
Superior temporal siikus*''
Superior temporal gyrus
, Posterior central gyrus
Postccntral sulcus
,,Supramarg. gyrusl
^Angular gyrus
''Superior parietal lobule
Interparielal sulcus
Trans, occipital sulcus
Sulcus lunatus
* \Infer tor temporal gyrus
\ Middle temporal sulcus
Middle temporal gyrus
Fig. 168. Sulci and gyri on the lateral aspect of the human cerebral hemisphere.
surface of the frontal lobe is composed of three convolutions, the superior,
middle, and inferior frontal gyri, separated from each other by the superior and
inferior frontal sulci. The inferior frontal gyrus, which in the left hemisphere
is also known as Broca's convolution, is subdivided by the two anterior rami of
the lateral sulcus into three parts, known as the orbital, triangular, and oper-
cular portions. The orbital part of the inferior frontal gyrus lies rostral to the
anterior horizontal ramus of the lateral sulcus; the triangular part is a wedge-
shaped convolution between the two anterior rami of that fissure; while the
opercular portion lies in the frontal operculum between the precentral sulcus
and the anterior ascending ramus of the lateral fissure.
The Temporal Lobe. Ventral to the lateral fissure is the long tongue-shaped
236 THE NERVOUS SYSTEM
temporal lobe which terminates rostrally in the temporal pole. The superior
temporal sulcus is a very constant fissure, which begins near the temporal pole
and runs nearly parallel with lateral cerebral fissure. Its terminal part turns
dorsally into the parietal lobe. The middle temporal sulcus, ventral to the pre-
ceding and in general parallel with it, is usually composed of two or more dis-
connected parts. The inferior temporal sulcus is located for the most part on
the basal surface of the temporal lobe. Dorsal to each of these fissures is a
gyrus which bears a similar name: the superior temporal gyrus, between the
lateral fissure and the superior temporal sulcus; the middle temporal gyrus, be-
tween the superior and middle temporal sulci; and the inferior temporal gyrus,
between the middle and inferior temporal sulci. The lateral fissure is very deep ;
and the surface of the superior temporal gyrus that bounds it is broad and marked
near its posterior extremity by horizontal convolutions, known as the transverse
temporal gyri. One of these, more marked than the others, has been called the
anterior transverse temporal gyrus or Heschl's convolution and represents the
cortical center for hearing (Fig. 174).
The Parietal Lobe. The postcentral sulcus runs nearly parallel with the
central sulcus and consists of two parts, the superior and inferior postcentral
sulci, which may unite with each other or with the inter parietal sulcus. Often
all three are continuous, forming a complicated fissure, as shown in Fig. 168.
The interparietal sulcus extends in an arched course toward the occiput and
may end in the transverse occipital sulcus. These four sulci are often included
under the term "interparietal sulcus." The interparietal sulcus proper is then
designated as the horizontal ramus.
The posterior central gyrus lies between the central and postcentral sulci.
The interparietal sulcus separates the superior parietal lobule from the inferior
parietal lobule. Within the latter we should take note of two convolutions:
the supramarginal gyrus, which curves around the upturned end of the lateral
fissure; and the angular gyrus, similarly related to the terminal ascending por-
tion of the superior temporal fissure.
The Occipital Lobe. Only a small part of the dorsolateral surface of the
hemisphere is formed by the occipital lobe. This is a triangular area at the
occipital extremity, bounded rostrally by a line joining the parieto-occipital
fissure and the preoccipital notch (Fig. 167). The transverse occipital fissure
may help to bound this area or may lie within it. Other inconstant sulci help
to divide it into irregular convolutions. Sometimes the visual area which lies
on the mesial aspect of this lobe is prolonged over the occipital pole to the lateral
THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
2 37
aspect. In this case a small semilunar furrow develops around it on the lateral
surface and is known as the sulcus lunatus (Fig. 168). This sulcus, called by
Riidinger the "Affenspalte," forms a conspicuous feature of the lateral surface of
the cerebral hemisphere in the lower Old World apes (Ingalls, 1914).
The Insula. The part of the cortex which overlies the corpus striatum lags
behind in its development and becomes overlapped by the surrounding pallium.
The cortex, which thus becomes hidden from view at the bottom of the lateral
fissure, forms in the adult a somewhat conical mass called the insula or island of
Reil (Fig. 169). Its base is surrounded by a limiting furrow, the circular sulcus,
which is, however, more triangular than circular, and in which we may recognize
three portions: superior, inferior, and anterior. The apex of this conical lobe
Parietal lobe
,/ Central sulcus of insula
Circular sulcus
, Frontal lobe
Occipital lobe x
V?- Short gyri of insula
Temporal lobe i ong gyrus O f i nsu la
Fig. 169. Lateral view of the human cerebral hemisphere with the insula exposed by removal of
the opercula. (Sobotta-McMurrich.)
is known as the limen insulce; and the remainder is subdivided by an oblique
groove (sulcus centralis insulae) into the long gyrus of the insula and a more
rostral portion, which is again subdivided into short gyri.
The Operculum. As the adjacent portions of the pallium close over the
insula (Fig. 164) they form by the approximation of their margins the three
rami of the lateral fissure. These folds constitute the opercula of the insula.
Each of the three surrounding lobes takes part in this process; and we may
accordingly recognize a, frontal, a temporal, and a parietal operculum (Fig. 166).
At this point it will be instructive to examine the lateral surface of the cerebral
hemisphere of the sheep. It will be seen that the region which corresponds to
the insula is on a level with the general surface of the hemisphere; no opercula
have developed, and the lateral sulcus is only a shallow groove (Fig. 173).
2 3 8
THE NERVOUS SYSTEM
THE MEDIAN AND BASAL SURFACES
The occipital lobe comes more nearly being a structural and functional
entity than any of the other lobes. It corresponds in a general way to the
"regio occipitalis" as outlined by Brodman (Figs. 216, 217), and it is probably
all concerned directly or indirectly with visual processes. We have seen that
it forms a small convex area on the lateral surface near the occipital pole;
and we now note that it is continued on to the medial surface of the hemi-
sphere, where it forms a somewhat larger triangular field between the parieto-
occipital and anterior portion of the calcarine fissure dorsorostrally and the
Sulcus cinguli
Sulcus of corpus callosum
Sup. fronta
gyrus'--
Frontal par. of
stilcus cinguli
Frontal pole
Genu of corp. cat.'
Septum pellucidum
Rost. of corpus callosum
Anterior parolfactory sulcus / / / ^
Par olfactory area < I
Temporal pole jUncus
Anterior commissure Fimbria
Hippocampal gyms
J3ody of corpus callosum
' Paracentral lobule
-' ' / Central sulcus
Marginal portion of sulcus cinguli
Precuneus
'Column of fornix
-'Subparietal sulcus
'.Cms of fornix
..- Parieto-occip. fis.
Splen. of corp. cal.
\.,-lsth. of gyms
fornicatus
j Cuneus
Calcarine
Ussure
'Occipital pole
' Lingual gyms
Inferior temporal gyrus
Inferior temporal sulcus
Fusiform gyrus
Collateral fissure
Fasciola cinerea
Fig. 170. Human cerebral hemisphere seen from the medial side. The brain has been
divided in the median plane and part of the thalamus has been removed along with the mesen-
cephalon and rhombencephalon. (Sobotta-McMurrich.)
collateral fissure ventrally. On this aspect of the brain it includes two constant
and well-defined convolutions: the cuneus and the lingual gyrus (Figs. 170,
171).
The calcarine fissure begins ventrally to the splenium of the corpus callosum
and extends toward the occipital pole, arching at the same time somewhat
dorsally. It consists of two portions. The rostral part, the calcarine fissure
proper, is deeper, more constant in form and position, and phylogenetically
much older than the rest, and produces the elevation on the wall of the lateral
ventricle known as the calcar avis (Fig. 181). This part terminates at the point
THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 239
where the calcarine is joined by the parieto-occipital fissure. The other portion,
sometimes called the ''posterior calcarine sulcus," arches downward and back-
ward from this junction toward the occipital pole, and occasionally cuts across
the border of the hemisphere to its dorsolateral surface. The parieto-occipital
fissure, which is really a deep fossa with much buried cortex at its depth, appears
to be the direct continuation of the rostral part of the calcarine fissure. It cuts
through the dorsal border of the hemisphere somewhat nearer to the occipital
pole than to the central sulcus. These fissures form a Y-shaped figure whose
stem is the calcarine fissure and whose two limbs are the parieto-occipital fissure
and the "posterior calcarine sulcus." If the fissures are opened up the stem is
seen to be marked off from the two limbs by buried annectant gyri.
The cuneus is a triangular convolution with apex directed rostrally, which
lies between the diverging parieto-occipital and calcarine fissures. The rest of
the medial surface of the occipital lobe belongs to the lingual gyrus, which lies
between the calcarine and collateral fissures.
The remaining sulci and gyri on the median and basal surfaces may now be
briefly described.
The sulcus of the corpus callosum (sulcus corporis callosi) begins ventrally to
the rostrum of the corpus callosum, encircles that great commissure on its con-
vex aspect, and finally bends around the splenium to become continuous with
the Mppocampal fissure (Fig. 171). The latter is a shallow groove, which runs
from the region of the splenium of the corpus callosum toward the temporal
pole near the dorsomedial border of the temporal lobe. It terminates in the
bend between the hippocampal gyrus and the uncus.
The sulcus cinguli (callosomarginal fissure) begins some distance ventral
to the rostrum of the corpus callosum and follows the arched course of the
sulcus of the corpus callosum, from which it is separated by the gyrus cinguli.
It terminates by dividing into two branches. One of these, the sub parietal
sulcus, continues in the direction of the sulcus cinguli and ends a short distance
behind the splenium. The other, known as the marginal ramus, turns off at a
right angle and is directed toward the dorsal margin of the hemisphere. A side
branch, directed florsally, is usually given off from the main sulcus some dis-
tance rostral to its bifurcation, and is known as the paracentral sulcus.
The collateral fissure begins near the occipital pole and runs rostrally, sepa-
rated from the calcarine and hippocampal fissures by the lingual and hippo-
campal gyri. It is sometimes continuous with the rhinal fissure. The latter
separates the terminal part of the hippocampal gyrus, which belongs to the archi-
240
THE NERVOUS SYSTEM
pallium, from the rest of the temporal lobe, and is a very conspicuous fissure in
most mammalian brains (Fig. 83) .
Convolutions. Dorsal to the corpus callosum is the gyrus cinguli between
the sulcus of the corpus callosum and the sulcus cinguli. The superior frontal
gyrus is continued over the dorsal border of the hemisphere from the dorso-
lateral surface and reaches the sulcus cinguli. Surrounding the end of the
central sulcus is a quadrilateral convolution, known as the paracentral lobule.
It is bounded by the sulcus cinguli, its marginal ramus and the paracentral
sulcus. Another quadrilateral area, known as the precuneus, is bounded by
the parieto-occipital fissure, the subparietal sulcus, and the marginal ramus of
the sulcus cinguli. The hippocampal gyrus lies between the hippocampal fissure
Superior frontal gyrus
Sulcus of corpus callosum ,- x
Gyrus cinguli -.
Sulcus cinguli -~,,^
Corpus callosum-,,^
Gyrus fornicatus -
Frontal lobe-
Post. par olfactory sulcus --
Parolfactory area-
Ant, par olfactory sulcus'''
Temporal lobe '
S. centralis
Paracentral sulcus
Paracentral lobule
- - Parietal lobe
*- Marginal ramus
__..-- Precuneus
vV Subparietal sulcus
^'\- - - Parieto-occipital fissure
- Cuneus
-- Calcarine fissure
- Lingual gyrus
, . Isthmus of gyrus
fornicatus
" Hippocampal fissure
Rhinal fissure '
Uncus ' Hippocampal gyrus J
Inf. temporal gyrus
Collateral fissure
Fusiform gyrus
Inferior temporal sulcus
Fig. 171. Diagram of the lobes, sulci, and gyri on the medial aspect of the human cerebral
hemisphere.
dorsally and the collateral and rhinal fissures ventrally. Its rostral extremity
bends around the hippocampal fissure to form the uncus. It is connected with
the gyrus cinguli by a narrow convolution, the isthmus of the gyrus fornicatus.
Under the name gyrus fornicatus it has been customary to include the gyrus
cinguli, isthmus, hippocampal gyrus, and uncus. Between the collateral fissure
and the inferior temporal sulcus is the fusiform gyrus which lies on the basal
surface of the temporal lobe in contact with the tentorium of the cerebellum
(Figs. 170, 172).
It has been customary to apportion parts of the medial and basal surfaces
of the cerebral hemisphere to the frontal, parietal, occipital, and temporal
lobes, as indicated in Fig. 171. According to this scheme the gyrus fornicatus
THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
241
stands by itself and is sometimes designated as the limbic lobe. This plan of
subdivision, which was based on the erroneous belief that all portions of the
gyrus fornicatus belonged to the rhinencephalon, should be abandoned. A
simpler and more logical arrangement assigns the hippocampal gyrus and uncus
to the temporal lobe and divides the gyrus cinguli between the frontal and
parietal lobes.
Longitudinal fissure of cerebrum
/ Frontal pole
rectus
^Olfactory sulcus
Orbital sulci
Olfactory trigone
Mammillary body
.-Uncus
L Middle temporal sulcus
Base of cerebral peduncle
Substantia nigra
Optic chiasma
Orbital gyri
Anterior perforated substance^
Temporal pole._
Lateral cerebral
(Sylvian) fissure
Middle temporal sulcus-..
Tuber cinereum-
I'VE* "*TWk TjiW ""* 3* !^^fc ^^^^B^^^T m j^s t ^H9T
^Inferior temporal gyrus
Fusiform gyrus
Hippocampal gyrus
Corpus quadrigeminum
Isthmus of gyms fornicatus
Lingual gyrus
"Gyrus cinguli
\ Splenium of corpus callosum
\ Parieto-occipital fissure
Occipital pole
Fig. 172. Basal aspect of the human cerebral hemisphere. (Sobotta-McMurrich.)
The basal surface of the hemisphere (Fig. 172) consists of two parts: (1)
the ventral surface of the temporal lobe, whose sulci and gyri have been de-
scribed in a preceding paragraph, and which rests upon the tentorium cerebelli
and the floor of the middle cranial fossa; and (2) the orbital surface of the frontal
lobe resting upon the floor of the anterior cranial fossa. The latter surface
presents near its medial border the olfactory sulcus, a straight, deep furrow,
directed rostrally and somewhat medially, that lodges the olfactory tract and
bulb. To its medial side is found the gyrus rectus. The remainder of the
orbital surface of the frontal lobe is subdivided by irregular orbital sulci into
equally irregular orbital gyri.
16
Hippocampal fissure
Collateral fissure'
Inferior temporal sulcus'
Cerebral aqueduct
Collateral fissure
CuneuS
242
THE NERVOUS SYSTEM
From the foregoing account it will be apparent that almost the entire sur-
face of the human cerebral hemisphere is formed by neopallium. Of the parts
already described only the uncus and adjacent part of the hippocampal gyrus
belong to the archipallium. Other superficial portions of the rhinencephalon,
such as the olfactory bulb, tract and trigone, and the anterior perforated sub-
stance, will be described in connection with the hidden parts of the rhinen-
cephalon in Chapter XVII.
Suprasylvian fissure
Cerebral hemisphere ^
Cerebellum
Postmedian lobule K,
Ansiform lobule'r-
Parafiocculus\~
Paramedian lobule*' -
Flocculus 1
; Lateral fissure
Insula
Chorioid plexus of
fourth ventricle
XII'
XI'
X // / / /
1X''/ VIII; : V 'IV /
Olive VII '; VI
Trapezoid body
Pan*
Rhinal Opic
fissure fissure
I Mammillary body
Hippocampal gyrus
Cerebral peduncle
\ Olfactory bulb
Lateral olfactory gyrus
Fig. 173. Lateral view of the sheep's brain.
The surface form of the cerebral hemisphere of the sheep is illustrated in
Figs. 83, 84, and 173. On these figures are indicated the names of the chief
sulci and gyri. It will be of interest to note the position of the motor cortex
in the sheep as given in Fig. 82. Since this corresponds to the precentral gyrus
in man, it will be seen that there is little in the sheep's brain to correspond to the
rostral part of the frontal lobe in man.
CHAPTER XVI
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
WHEN a horizontal section is made through the cerebral hemisphere at the
level of the dorsal border of the corpus callosum the central white substance
will be displayed in its maximum extent and will appear as a solid, semioval
mass, known as the centrum semiovale (Figs. 174, 175). It will also be apparent
that lamellae extend from this central white substance to form the medullary
centers of the various convolutions, and that over this entire mass the cortex is
spread in an uneven layer, thicker over the summit of a convolution than at
the bottom of a sulcus. This medullary substance is composed of three kinds
of fibers: (1) fibers from the corpus callosum and other commissures joining the
cortex of one hemisphere with that of the other; (2) fibers from the internal cap-
sule, uniting the cortex with the thalamus and lower lying centers; and (3)
fibers running from one part of the cortex to another within the same hemi-
sphere (p. 296).
The Corpus Callosum. At the bottom of the longitudinal fissure of the
cerebrum is a broad white band of commissural fibers, known as the corpus
callosum, which connects the neopallium of the two hemispheres. While the
medial portion of this commissure is exposed in the floor of the longitudinal
fissure, its greater part is concealed in the white center of the hemisphere where
its fibers radiate to all parts of the neopallium, forming the radiation of the
corpus callosum. When examined in a median sagittal section of the brain the
corpus callosum is seen to be arched dorsally and to be related on its ventral
surface to the fornix and septum pellucidum (Figs. 84, 158, 170). The latter
consists of two thin membranous plates, stretched between the corpus callosum
and the fornix and separated by a narrow cleft-like space, the cavum septi
pellucidi (Fig. 177). If the septum has been torn away it will be possible to
look into the lateral ventricle and see that the corpus callosum forms the roof
of a large part of that cavity. At its rostral extremity it curves abruptly toward
the base of the brain, forming the genu, and then tapers rapidly to form the
rostrum. The latter is triangular in cross-section, with its edge directed toward
the anterior commissure to which it is connected by the rostral lamina. The
243
244
THE NERVOUS SYSTEM
body of the corpus callosum (truncus corporis callosi), arching somewhat dor-
sally, extends toward the occiput and terminates in the splenium, a thickened
rounded border situated dorsal to the pineal body and corpora quadrigemina.
Related to the concave or ventral side of the corpus callosum are the fornix,
septum pellucidum, lateral ventricles, tela chorioidea of the third ventricle, and
the pineal body (Fig. 170).
Genii of corpus callosum
Cingulum (cut) ~^pF
Corpus callo-...
sum
Centrum semi-
ovale
Medial longi-..
ludinal stria
Cingulum (cut) J
Splenium of _,,
corp. callosum"
Frontal part of
' radiation of
corp. callosum
Intersection of
fibers from cor-
. pus callosum
and corona
radiata
.Superior longi-
tudinal fas-
ciculus
Radiation of
corp. callosum
^Transverse tem-
poral gyri
Optic radiation
Occipital part of
radiation of
corp. callosum
Fig. 174. Dissection of the human telencephalon to show the radiation of the corpus callosum.
Dorsal view.
Turning again to the dorsal aspect of the corpus callosum, a careful inspec-
tion will show that at the bottom of the great longitudinal fissure it is covered
by a very thin coating of gray matter, continuous with the cerebral cortex in
the depths of the sulcus of the corpus callosum (Figs. 174, 175). This is a rudi-
mentary portion of the hippocampus and is known as the supracallosal gyrus or
indusium griseum. In this gray band there are embedded delicate longitudinal
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
245
strands of nerve-fibers. Two of these, placed close together on either side of
the median plane, are known as the medial longitudinal stria. Further lateral-
ward on either side, hidden within the sulcus of the corpus callosum, is a less
well -developed band, the lateral longitudinal stria.
The corpus callosum is transversely striated and is composed of fibers that
pass from one hemisphere to the other. By dissection these may be followed
into the centrum semiovale, where they constitute the radiation of the corpus
Genii of corpus callosum
Medial longitudinal stria
- Hippocampal rudiment
~;_-Body of corpus callosum
Radiation of corpus callosum
~ \--Corona radiata
Intersection of corona ra-
/A diata and radiation of
A corpus callosum
Lateral longitudinal stria
Splenium of corpus callosum
Fig. 175. Dissection of the telencephalon of the sheep to show the radiation of the corpus cal-
losum. Dorsal view.
callosum and intersect those from the internal capsule in the corona radiata
(Figs. 174, 175). The fibers of the genu sweep forward into the frontal lobe,
constituting the frontal part of the radiation. Fibers from the splenium bend
backward toward the occipital pole, forming the occipital part of the radiation
or forceps major. In the human brain fibers from the body and splenium
of the corpus callosum sweep outward over the lateral ventricle, forming the
roof and lateral wall of its posterior horn and the lateral wall of its inferior
cornu. Here they constitute a very definite stratum called the tapetum.
246
THE NERVOUS SYSTEM
THE LATERAL VENTRICLE
When the corpus callosum and its radiation are cut away a cavity, known
as the lateral ventricle, is uncovered. It is lined by ependyma, continuous with
the ependymal lining of the third ventricle by way of the interventricular for-
amen. This cavity, which contains cerebrospinal fluid, varies in size in differ-
ent parts, and in some places is reduced to a mere cleft between closely apposed
walls. The shape of the ventricle is highly irregular (Fig. 176). As constit-
uent parts we recognize a central portion, anterior and inferior horns, and in
man also a posterior horn. The latter part develops rather late in the human
fetus as a diverticulum from the main cavity.
Third ventricle
Ant. horn
[Lateral ventricle
Inf. horn) Central P art
y Interventricular for.
' " Optic recess
* ' Infundibulum
\ > Third ventricle
\^ Inf. horn
\^ Suprapineal recess
x Cerebral aqueduct
Fourth ventricle
Fourth ventricle
Post, horn
A B
Fig. 176. Two views of the brain ventricles of man: A, Dorsal view; B, lateral view.
The anterior horn, or cornu anterius, is the part which lies rostral to the
interventricular foramen. Its roof and rostral boundary are formed by the
corpus callosum. Its medial wall is vertical and is formed by the septum pellu-
cidum, which is stretched between the corpus callosum and the fornix (Figs.
177, 178). The sloping floor is at the same time the lateral wall, and is formed
by the head of the caudate nucleus, which bulges into the ventricle from the
ventrolateral side. In frontal section the cavity has a triangular outline; and
in such a section its walls and the relation which they bear to the rest of the brain
can be studied to advantage (Fig. 186).
The central part or body of the lateral ventricle extends from the inter-
ventricular foramen to the splenium of the corpus callosum, where in man the
cavity bifurcates into posterior and inferior horns. The roof of the central
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
247
part is formed by the corpus callosum, and the medial wall by the septum pellu-
cidum. The floor, which slants to meet the roof at the lateral angle, is com-
posed from within outward of the following structures: the fornix, chorioid
plexus, lateral part of the dorsal surface of the thalamus (in man, but not
in the sheep), the stria terminalis, vena terminalis, and the caudate nucleus
(Figs. 177-180, 188). The caudate nucleus tapers rapidly as it is followed
from the anterior horn into the body of the ventricle (Fig. 177). The cavity
Longitudinal fissure of cerebrum
Lamina of septum pellucidum /
Column of fornix
Caudate nucleus
Interventricular foramen
Thalamus
Body of fornix
Chorioid plexus
Transverse fissure of /
cerebrum
Rostrum of corpus callosum
' ^Corpus callosum
/ Cavity of septum pellucidum
Anterior horn of lateral ventricle
/Caudate nucleus
Chorioid plexus of lateral
ventricle
, Terminal stria
Central portion of
lateral ventricle
- Chcrioid glomus
- Cms of fornix
.-Inferior horn of
lateral ventricle
Splenium of corpus callosum
* Posterior horn of lateral ventricle
Calcarine fissure
Cerebellum
Fig. 177, Dissection of the human telencephalon. The corpus callosum has been partly removed,
and the lateral ventricles have been exposed. Dorsal view. (Sobotta-McMurrich.)
is lined throughout by an ependymal epithelium, indicated in red in Fig. 155.
Between the caudate nucleus and the fornix this layer of ependyma constitutes
the entire thickness of the wall of the hemisphere. In man, where the fornix
and caudate nucleus are more widely separated than in the sheep, this epithelial
membrane rests upon the thalamus and becomes adherent to it as the lamina
affixa (Figs. 154, 155). At the margin of the fornix a vascular network from the
tela chorioidea, i. e., from the pia mater in the transverse cerebral fissure, is
248
THE NERVOUS SYSTEM
invaginated into the ventricle, pushing this epithelial layer before it and con-
stituting the chorioid plexus.
The posterior horn, or cornu posterius, extends into the occipital lobe of
the human brain, tapering to a point, and describing a gentle curve with con-
cavity directed medially (Figs. 177, 181).
The tapetum of the corpus callosum forms a thin but distinct layer in the
roof and lateral watt of the posterior horn, and is covered in turn by a thicker
layer of fibers belonging to optic radiation or radiatio occipitothalamica (Fig.
190). In the medial wall two longitudinal elevations may be seen. Of these,
Corpus callosum
Head of caudate
nucleus "
Body offornix--^
Fimbria of hippo-
campus
Hippocampus -
Splenium of corpus
callosum
Anterior horn of
lateral ventricle
Thick portion of
septum pellucidum
~ Lateral fissure
Interventricular
foramen
- Lateral ventricle
Fig. 178. Dissection of the telencephalon of the sheep to show the lateral ventricle and the
structures which form its floor. Dorsal view.
the more dorsal one is known as the bulb of the posterior horn (bulbus cornu),
and is formed by the occipital portion of the radiation of the corpus callosum
or forceps major. The other elevation, known as the calcar avis, is larger and
is produced by the rostral part of the calcarine fissure, which here causes a fold-
ing of the entire thickness of the pallium (p. 238).
The inferior horn, or cornu inferius, curves ventrally and then rostrally into
the temporal lobe (Fig. 181). The angle between the diverging inferior and
posterior horns is known as the collateral trigone. This horn lies in the medial
part of the temporal lobe and does not quite reach the temporal pole. The roof
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 249
is formed by the white substance of the hemisphere, and along its medial border
are the stria terminalis and tail of the caudate nucleus. At the end of the latter
Genii of corpus
callosum
Septum pellucidum
Thick portion of sep-
tum pellucidum
Hippocampus -9f~~
Inferior horn of,
lateral ventricle
-Head of caudate
nucleus
i- I nterventricular
foramen
"Chorioid fissure
ft -Fimbria of
hippocampus
Transverse fissure of cerebrum !
Tfialamus
Fig. 179.
\ Hippocampal commissure
Pineal body
Lateral ventricle --
Septum pellucidum
Thick portion of
septum pellucidum
Column of fornix
Thalamus I ',
Third ventricle i
Pineal body
\ Thalamus
\ Tania of thalamus
'Habenular trigone
'Genu of corpus
callosum
Head of caudate
nucleus
- I nterventricular
foramen
Fimbria of hippo-
campus
- Inferior horn of
lateral ventricle
" Hippocampus
Fig. 180.
Figs. 179 and 180. Dissections of the rostral part of the sheep's brain to show the relation
of the lateral ventricles, fornix, fimbria, and hippocampus to the transverse fissure, thalamus, and
third ventricle. Dorsal views. In Fig. 180 a triangular piece, including portions of the fornix,
fimbria, and hippocampus, has been removed.
the amygdaloid nucleus bulges into the terminal part of the inferior horn (Fig.
185). The floor and medial wall of the inferior horn are formed in large part
250
THE NERVOUS SYSTEM
by the following structures, named in their order from within outward: the
fimbria, hippocampus, and (in man) the collateral eminence (Figs. 181, 182,
189). Upon the fimbria and hippocampus there is superimposed the chorioid
plexus (Fig. 183). The hippocampus is a long, prominent, curved elevation,
with whose medial border there is associated a band of fibers, representing a
continuation of the fornix and known as the fimbria. These parts will be de-
Lamina of septum pellucidum
Columns of fornix
Anterior tubercle of thala-
Uncus.
Hippocampal
digitations\
Hippocampal,.
gyrus
Collateral eminence
Fimbria of hippo-
campus .
Collateral trigone
Posterior commissure
Hippocampus
Calcar avis
Posterior horn of lateral ventricle
f Longitudinal fissure of cerebrum
Corpus callosum
, Cavity of septum pellucidum
Interventricular foramen
Anterior horn of lateral ventricle
Head of caudate nucleus
,Massa intermedia
, Third ventricle
, Habenular commissure
,-Habenular trigone
^Inferior horn of lateral
ventricle
Posterior horn of lat-
eral ventricle
Pineal body
Corpora quadrigemina
Vermis of cerebellum
Fig. 181. Dissection of the human brain to show the posterior and inferior horns of the lateral
ventricle. The body and splenium of the corpus callosum have been removed, as have also the body
of the fornix and the tela chorioidea of the third ventricle. A sound has been passed through the
interventricular foramina. Dorsal view. (Sobotta-McMurrich.)
scribed in connection with the rhinencephalon. The collateral eminence is an
elevation in the lateral part of the floor produced by the collateral fissure.
The thin epithelial membrane, described above as joining the edge of the
fornix with the caudate nucleus (Fig. 155), continues to unite these structures as
they both curve downward, the former in the floor, the latter in the roof, of the
inferior horn. A vascular plexus from the pia mater is invaginated into the
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 251
lateral ventricle along this curved line, carrying before it an epithalial covering
from this thin membrane. In this way there is formed the chorioid plexus of the
lateral ventricle (Figs. 183, 184). The line along which this imagination occurred
is the chorioid fissure; and when the plexus is torn away, the position of the
Lateral ventricle
Inter-ventricular foramen
Hippocampus'
Fimbria of hippocampus' / /
Body offornix
Optic tract /
Internal capsule
/ Olfactory bulb
I \ Rhinoccele
I Genu of corpus cattosum
I Body of corpus callosum
Septum pellucidum
Fig. 182. Dissection of the cerebral hemisphere of the sheep to show the lateral ventricle.
Lateral view.
fissure is indicated by an artificial cleft extending into the ventricle, which be-
gins at the interventricular foramen and follows the fornix and fimbria in an
arched course into the temporal lobe (Fig. 205).
Hippocampus Chorioid plexus of lateral ventricle
Fig. 183. Outline drawing from Fig. 182, to show the location of the chorioid plexus of the lateral
ventricle.
The chorioid plexus of the lateral ventricle (Figs. 183, 184, 188) is continuous
with that of the third ventricle at the interventricular foramen, from which
point it can be followed backward through the central part into the inferior
horn. It is coextensive with the chorioid fissure and is not found in the anterior
or posterior horns. It consists of a vascular network derived from the pia
252
THE NERVOUS SYSTEM
mater, and especially from that part of it enclosed in the transverse fissure and
known as the tela chorioidea of the third ventricle. It is covered throughout
Longitudinal fissure of cerebrum
Anterior horn of lateral
ventricle
Corpus striatum-, :
Interventricular for.
Columns offornix
Central portion of
lateral ventricle
Internal cerebral
veins
Chorioid vein
Chorioid artery
Inferior horn of
lateral ventricle
Collateral trigone
Posterior horn
Body of corpus callosum
.Lamina of septum pellucidum
-Cavity of sept, pellucidum
L .Lamina: of septum
pellucidum
-Vein of septum
pellucidum
Terminal vein
,-Thalamus
-J, --Corpus striatum
Lateral chorioid
2<jL - ' plexus
"^ / -Tel a chorioidea
of third ventricle
'Chorioid glomus
Calcar a^jis ll^^^B
Great cerebral vein Hippocampal Body of corpus Body offornix Crura offornix
commissure callosum
Fig. 184. Dissection of the human brain to show the tela chorioidea of. the third ventricle
and the hippocampal commissure. The body of the corpus callosum and the fornix have been
divided and reflected. Dorsal view, except that the ventral surfaces of the reflected corpus
callosum and hippocampal commissure are seen. (Sobotta-McMurrich.)
by a layer of epithelium of ependymal origin, which is adapted to every uneven-
ness of its surface (Fig. 155).
THE BASAL GANGLIA OF THE TELENCEPHALON
There are four deeply placed masses of gray matter within the hemisphere,
known as the caudate, lentiform and amygdaloid nuclei, and the claustrum. The
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
253
two former, together with the white fascicles of the internal capsule which
separate them, constitute the corpus striatum (Fig. 185).
The caudate nucleus (nucleus caudatus) is an elongated mass of gray matter
bent on itself like a horseshoe, and is throughout its entire extent closely re-
Caudate nucleus
Thalamos
Lenticular nucleus
Amygdaloid nucleus
Caudate nucleus
Thalamus
Tail of caudate nucleus
Internal capsule
Lenticular nucleus
Caudate nucleus
Thalamus
Tail of caudate nucleus
Internal capsule
Fig. 185. Diagrams of lateral view and sections of the nuclei of the corpus striatum with the
internal capsule omitted. A and B below represent horizontal sections along the lines A and B
in the figure above. The figure also shows the relative position of the thalamus and the amygda-
loid nucleus. (Jackson- Morris.)
lated to the lateral ventricle (Figs. 91, 177, 178, 186, 187, 188, 191). Its swol-
len rostral extremity or head is pear shaped and bulges into the anterior horn of
the lateral ventricle. The remainder of the nucleus is drawn out into a long,
slender, highly arched tail. In the floor of the central part of the ventricle the
head gradually tapers off into the tail, which finally curves around into the roof
254
THE NERVOUS SYSTEM
of the inferior horn and extends rostrally as far as the amygdaloid nucleus.
Because of its arched form it will be cut twice in any horizontal section which
passes through the main mass of the corpus striatum, and in any frontal section
through that body behind the amygdaloid nucleus (Figs. 185, 189, 191). The
head of the caudate nucleus is directly continuous with the anterior perforated
substance; and ventral to the anterior limb of the internal capsule it is fused with
the lentiform nucleus (Fig. 186).
The lentiform or lenticular nucleus (nucleus lentiformis) is deeply placed
in the white center of the hemisphere and intervenes between the insula, on the
medialis.
Stria
longitu
dinalis I lateralis
Corpus ._
callosum
Caput nuclei . j
caudati *
I
Claustrum _j.
Capsula
externa
Capsula
interna
Nucleus lentifor-
mis (Putamen)
Fibers from
the tractus
olfactorius
Gyrus rectusr
Fissura longi
tudinalis
cerebri
Polus temporalis -'
Fissura longitudi-
nalis cerebri
i_..Gyrus cinguli
Sulcus corporis
callosi
Cornu anterius
ventriculi
lateralis
..Vena septi
pelluciiti
_ Septum
pellucidum
~.Fissura cerebri
lateral is(Sylvii)
., Rostrum cor-
"poris callosi
""--- Gyrus sub-
callosus
.Area parolfac-
toria (Brocae)
Fissura cerebri
lateralis (Sylvii)
Fig. 186. Frontal section of the human brain through the rostral end of the corpus striatum and
the rostrum of the corpus callosum. (Toldt.)
one hand, and the caudate nucleus and thalamus on the other (Figs. 185, 191,
194). In shape it bears some resemblance to a biconvex lens. Its lateral,
moderately convex surface is nearly coextensive with the insula from which it
is separated by the claustrum. Its ventral surface rests upon the anterior per-
forated substance and the white matter forming the roof of the inferior horn of
the lateral ventricle (Figs. 187-189). Its sloping medial surface is closely
applied to the internal capsule. The lentiform nucleus is not a homogeneous
mass, but is divided into three zones by internal and external medullary lamina.
The most lateral zone is the largest and is known as the putamen. The two
medial zones together form the globus pallidus.
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
2 55
Nucleus caudatus
(Caput)
Capsula interna^
(Pars frontalis
_. Fissura longitudinalis
cerebri
f . Corpus callosum
Cornu anterius
ventriculi
laterlais
[_,. Plexus chorio-
ideus ventriculi
lateral!*
^JC-^ Septum pellu-
cidum
Foramen inter--
ventriculare
(Monroi)
Substantia per-
forata anterior'
Uncus''
- Fissura cerebri
lateralis(Sylvii)
insulae
ssus opti-
cus ventriculi
tertii
"~ Tractus opticus
Chiasma opti-
-- cum (posterior
part;
^ Commissura inferior
(Citddeni)
Fig. 187. Frontal section of the human brain through the anterior commissure. (Toldt.)
Ventriculus lateralis x
(Pars centralis)
Plexus chorioideus,
ventriculi lateralis ~
Nucleus
caudatus""^--^
Massa inter-,
media
Capsula interna
Putamen~
Nucleu
lenti-
formis
pallidus
Capsula externa..
Claustrum.^^
Ansa peduncu-
laris
Tractus opticus- ~
Pedunculus. tha-^-^'
lami inferior
Cornu inferius ve:
triculi lateralis
Digitationes#
hippocampi
N. oculomotorius
Ansa lenticularis
j*if_. Nucleus hypo-
5, thalamicus
I (Corpus Luysi)
Substantia
nigra
Basis pedunculi
Corpus
~~-~.mamillare
Fossa inter-
- peduncularis
-Pens (Varoli)
Fig. 188. Frontal section of the human brain through the mammillary bodies. (Toldt.)
The putamen is larger than the globus pallidus and is encountered alone in
frontal sections through either the rostral or caudal extremities of the corpus
striatum (Fig. 189), and also in horizontal sections above the level of the globus
256
THE NERVOUS SYSTEM
pallidus (Fig. 191). It is fused rostrally with the caudate nucleus, which it
resembles in color and structure.
The globus pallidus is lighter in color and is subdivided into two parts, of
which the medial is the smaller. Both parts are traversed by many fine
white fascicles from the medullary laminae.
Especially in the anterior part of the internal capsule bands of gray sub-
stance stretch across from the lentiform to the caudate nucleus, producing a
striated appearance (Fig. 187). This appearance, which is accentuated by the
medullary laminae and the finer fiber bundles in the lentiform nucleus, makes
Tela chorioidea
ventriculi tertii
Capsula in
Nucleus
habenulae
Cauda nuclei
caudati
Tractus opticus ~;
Fimbria hippo-
campi
Fascia dentata
hippocampi
Pedunculus cerebri
V cerebri interna.
Plexus chorio-
'ideus ventriculi
tertii
Commissura
habenularum
Commissura
posterior
Aditus ad aquae-
ductum cerebri
Fasciculus retro-
flexus(Meynerti)
Cornu inferius
ventriculi
lateralis
...Nucleus rubcr
Nucleus hypo-
thalamicus
(Corpus Luysi)
- x - Substantia nigra
Pons (Varoli)
Recessus posterior fossae interpeduncularis/
Fig. 189. Frontal section of the human brain through the rostral part of the pons. (Toldt.)
the term corpus striatum an appropriate name to apply to the two nuclei and
the internal capsule, which separates them.
The claustrum is a thin plate of gray substance, which, along with the white
matter in which it is embedded, separates the putamen from the cortex of the
insula. Its lateral surface is somewhat irregular, being adapted to the convolu-
tions of the insula, with which it is coextensive (Figs. 188, 191). Its concave
medial surface is separated from the putamen by a thin lamina of white matter,
known as the external capsule. By some authorities the claustrum is thought
to be a detached portion of the lentiform nucleus, while others believe that it
has been split off from the insular cortex. It is probable that neither of these
views is strictly correct. However, according to the recent work of Elliot
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
257
Smith (1919), the claustrum, putamen, amygdaloid nucleus, and the greater
part of the caudate nucleus are pallial derivatives and are closely related mor-
phologically to the neopallium; while the globus pallidus is the representative
in the mammalian brain of the corpus striatum of lower forms, as seen in the
shark (Fig. 9).
The Amygdaloid Nucleus. In the roof of the terminal part of the inferior
ventricular horn, at the point where the tail of the caudate nucleus ends, there
is located a small mass of gray matter, known as the amygdaloid nucleus (Fig.
Radiatio corporis N
callosi
Bui bus cornu-.
posterioris
Calcar
Hippocampus -^ '' ,
* - - /,' s
Corpora
tjuadrigemina -- .
Nucleus
.colliculi
inferioris
Aquaeductus /_/.
cerebri
?.*,
Nucleus n
trochlearis ... .... x
T^ I ''/' : 'j ''
Fasciculus _.
longitudinalis
medial is
Cerebellum--'
Brachium pontis --'
Flocculus
Pyramis medullae oblongatae.-'"
, Splenium cor-
poris callosi
Tela chorio-
idea ven-
triculi tertii
T Corpus
>'' pineale
Cornu posle-
,'' rius-ventri-
culi lateralis
^, Glomus
chorioideum
yA-r- J ':'" ^--^--- Tapetum
_ ...v.t Radiatio occi-
i \\ \\' pitothalamica
Eminentia
collateralis
Fissura
*. \;> collateralis
Lemniscus
' J ^ lateralis
""J Brachium con-
junct! vum
-- Stratum griseum
centrale
~~- Lemniscus medialis
N. vagus
Fig. 190. Frontal section of the human brain through the splenium of the corpus callosum. View
into the posterior horn of the lateral ventricle. (Toldt.)
185). It is continuous with the cerebral cortex of the temporal lobe lateral to
the anterior perforated substance (Fig. 198; Landau, 1919).
The external capsule is a thin lamina of white matter separating the claus-
trum from the putamen. Along with the internal capsule it encloses the lenti-
form nucleus with a coating of white substance.
THE INTERNAL CAPSULE
The internal capsule is a broad band of white substance separating the
lentiform nucleus on the lateral side from the caudate nucleus and thalamus on
the medial side (Figs. 191, 192). In a horizontal section through the middle
17
2 5 8
THE NERVOUS SYSTEM
of the corpus striatum it has the shape of a wide open V. The angle, situated
in the interval between the caudate nucleus and the thalamus, is known as the
Truncus corporis callosi.
Septum pellucid
Corpus fornii
Genu corporis callosi
entriculi lateralis
:lei caudati
nna fornicis
^apsula interna
.Insula
,Capsula externa
Claustrum
Putamen
.Globus
pallidus
Nucleus
lent!-
Glomus chorio-
ideum
Radiatio occi-
pitothalamica
(Gratioleti)
Splenium corporis ca
Massa inter-
media
Ventriculus
tertius
L.Stria medullaris
thalarni
- Nucleus
habenulae
Habenula
-Cauda nuclei
- caudati
Fimbria hippo-
campi
^Corpus pineale
^Hippocampus
Eminentia
collaterals
Calcar avis
Cornu posterius
'entriculi lateralis
Fissura calcarina
Fig. 191. Horizontal sections of the human brain through the internal capsule and corpus
striatum. The section on the right side was made 1.5 cm. farther ventralward than that on the
left. (Toldt.)
genu. From this bend the frontal part or anterior limb of the internal capsule
extends laterally and rostrally between the thalamus and the head of the caudate
nucleus; while the occipital part or posterior limb of the internal capsule extends
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
259
laterally and toward the occiput between the lentiform nucleus and the thala-
mus.
The anterior limb of the internal capsule, intervening between the caudate
and lentiform nuclei, is broken up by bands of gray matter connecting these
two nuclei. It consists of corticipetal and corticifugal fibers. The former
belong to the frontal stalk of the thalamus or anterior thalamic radiation from the
lateral nucleus of the thalamus to the cortex of the frontal lobe. The cortici-
Septum pellucidum ,^
Fornix^
Chorioid fissure
Third ventricle ^
Thalamus^ s^
/ ~">
Habenular trigone -^K
Habenular commis- ^U
sure I
Transverse fissure ^|
Pineal body
Inferior horn of _
lateral ventricle
Superior colliculus
'::
-' Genu of corpus callosum
.^Anterior horn of lateral ventricle
, Anterior limb of internal cap-
sule
fe.- Head of caudate nucleus
~~Insnla
r- External capsule
-^ Lentiform nucleus
'Claitstrum
\ N " % *. Genu of internal
capsule
\
\"y ^Posterior limb of
internal capsule
v/ Chorioid fissure
\j& ; * ^Fimbria of hippocampus
\) "Hippocampus
^-^Cerebellum
Medulla oblongata
Fig. 192. Horizontal section through the sheep's brain, passing through the internal capsule and
corpus striatum.
fugal fibers form the frontopontine tract from the cortex of the frontal lobe to
the nuclei pontis (Fig. 193).
The posterior limb of the internal capsule intervenes between the thalamus
and the lentiform nucleus, and bends around the posterior end of the latter
on to its ventral surface (Fig. 194). It accordingly consists of three parts,
designated as lenticulothalamic, retrolenticular, and sublenticular. The lentic-
ulothalamic part consists of fibers belonging to the thalamic radiation intermingled
with others representing the great efferent tracts which descend from the cere-
260
THE NERVOUS SYSTEM
bral cortex (Fig. 193). Of these, the corticobulbar tract to the motor nuclei of
the cranial nerves occupies the genu, and the cor tico spinal tract the adjacent
portion of the posterior limb. The fibers of the corticospinal tract are so ar-
Caudate nucleus
Frontopontine tract
Anterior thalamic radiation
^Corticobulbar tract
'Globus pallidus
^-Corticorubral tract
- Corticospinal tract (arm)
-Corticospinal tract (leg)
-Putamen
- Thalamic radiation (sensory fibers)
Auditory radiation
Thalamus
Optic radiation
Fig. 193. Diagram of the internal capsule.
ranged that those for the innervation of the arm are nearer the genu than those
for the leg. Accompanying the corticospinal tract are descending fibers from
the cortex of the frontal lobe to the red nucleus, the corticorubral tract. Those
Coronal fibers from posterior limb of
internal capsule
Coronal fibers from anterior L^
limb of internal capsule
Lentiform nucleus
Coronal fibers from retro-
lenticular part of inter-
nal capsule
Coronal fibers from sublenticular part
of internal capsule
Anterior commissure-'
Ansa peduncularis''
Fig. 194. The lentiform nucleus and the corona radiata dissected free from the left human
cerebral hemisphere. Lateral view.
fibers of the thalamic radiation which run to the posterior central gyms and con-
vey general sensory impulses from the lateral nucleus of the thalamus are sit-
uated behind the corticospinal tract. The retrolenticular part of the internal
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES
26l
capsule rests upon the lateral surface of the thalamus behind the lentiform
nucleus and contains: (1) the optic radiation from the pulvinar and lateral
geniculate body to the cortex in the region of the calcarine fissure, and (2) the
acoustic radiation from the medial geniculate body to the transverse temporal
gyrus. The sublenticular part of the internal capsule lies ventral to the pos-
terior extremity of the lenticular nucleus and contains the temporopontine tract
from the cortex of the temporal lobe to the nuclei pontis.
Dissections of the Internal Capsule (Figs. 87, 88, 91, 194, 195). A large
part of the fibers of the internal capsule, including the corticopontine, cortico-
bulbar, and corticospinal tracts, are continued as a broad thick strand on the
ventral surface of the cerebral peduncle, with which we are already familiar
Radiation of corpus cal-
losum forming roof
of lateral ventricles
A nterior limb of inter
nal capsule (can- -^ d
date impression) ".^EJ
X
Frontal pole -
Posterior limb of internal capsule
/(thalamic impression)
/ Tapelum
Genii internal cap- ,--^<
side
Anterior commissure
Optic tract
Temporal lobe -
Basis pedunculi Optic radiation
Fig. 195. Dissection of the human cerebral hemisphere, showing the internal capsule exposed
from the medial side. The caudate nucleus and thalamus have been removed.
under the name basis pedunculi. By removing the optic tract, temporal lobe,
insula, and lentiform nucleus this strand can easily be traced into the internal
capsule where it is joined by many fibers radiating from the thalamus and
spreads out in a fan-shaped manner (Figs. 87, 88), forming a curved plate which
partially encloses the lentiform nucleus. As seen from the lateral side, the line
along which the fibers of the internal capsule emerge from behind the lentiform
nucleus forms three-fourths of an ellipse (Fig. 194). Beyond the lentiform nu-
cleus the diverging strands from the internal capsule, known as the corona
radiata, join the central white substance of the hemisphere and intersect with
those from the corpus callosum (Figs. 174, 238).
An instructive view of the internal capsule may also be obtained by remov-
262
THE NERVOUS SYSTEM
ing the thalamus and caudate nucleus from its medial surface. It is then seen
to bear the imprint of both of these nuclei, and especially of the thalamus; and
between the two impressions it presents a prominent curved ridge (Fig. 195).
This ridge is responsible for the sharp bend known as the genu, which is evi-
dent in horizontal sections at appropriate levels through the capsule. Many
broken bundles of fibers, representing the thalamic radiation, are seen enter-
ing the capsule upon its medial surface.
THE CONNECTIONS OF THE CORPUS STRIATUM AND THALAMUS
What is the function of the corpus striatum, and what connection does it
have with other parts of the nervous system? These questions, to which no
Caudate nucleus
Parietal stalk of thalamus
Corticospinal tract
Insula
Claustrum
Putamen
Globus pallidus
Ansa peduncularis
Red nucleus
\ )\ ;^x>\
Ansa lenticularis
Substantia nigra
Hypothalamic nucleus
Fig. 196. Diagram of the connections of the caudate and lenticular nuclei.
final answer can as yet be given, have recently become of great importance,
because of the frequency with which degeneration of the lentiform nucleus has
been found at autopsy in patients who have shown serious disturbances of the
motor mechanism (Wilson, 1912-1914). It seems probable that the corpus
striatum exerts a steadying influence upon muscular activity, the abolition of
which results in tremor during voluntary movement. The probable connec-
tions of the corpus striatum are indicated in Fig. 196. Striopetal fibers reach
the caudate nucleus from the anterior and medial nuclei of the thalamus (Sachs,
1909). According to Cajal, the corpus striatum also receives collaterals from
the corticospinal tract. Internuncial fibers join together various parts of the
corpus striatum. The majority of these seem to run from the caudate nucleus
THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 263
to the putamen, on the one hand, and from the putamen to the globus pallidus
on the other. The striofugal fibers arise, for the most part at least, in the globus
pallidus. They are collected into a bundle of transversely directed fibers, known
as the ansa lenticularis (Fig. 188), which is distributed to the thalamus, red
nucleus, hypothalamic nucleus, and substantia nigra. Other fibers belonging
to the same general system break through the ventral third of the internal
capsule to reach the thalamus (Wilson, 1914). The importance of the connec-
tion with the red nucleus is obvious, since by way of the rubrospinal and rubro-
reticular tracts the corpus striatum is able to exert its influence upon the pri-
mary motor neurons of the brain stem and spinal cord. The fibers to the sub-
stantia nigra have already been mentioned under the name strionigral tract
(p. 164). The impulses which travel along them are, in all probability, re-
layed through the substantia nigra to lower lying motor centers, although the
functions and connections of this large nuclear mass are still obscure.
The Thalamic Radiation. We are now in position to understand the course
and distribution of the fascicles, which unite the thalamus with the cerebral
cortex and which consist of both thalamocortical and corticothalamic fibers. This
thalamic radiation may be divided into four parts: the frontal, parietal, occip-
ital, and ventral stalks of the thalamus, which will now be traced as fasciculi,
without reference to the direction of conduction in the individual fibers.
The ventral stalk, or inferior peduncle of the thalamus, streams out of the
rostral portion of the ventral thalamic surface and is directed lateral ward under
cover of the lentiform nucleus. Some of these fibers belong to the ansa lentic-
ularis and run from the lentiform nucleus to the thalamus. The others, form-
ing a bundle known as the ansa peduncularis , runs lateralward ventral to the
lentiform nucleus and are distributed to the cortex of the temporal lobe and
insula (Fig. 196).
The frontal stalk, or peduncle of the thalamus, consists of fibers which run
through- the anterior limb of the internal capsule from the lateral thalamic
nucleus to the cortex of the frontal lobe (Fig. 193), and in small part to the cau-
date nucleus also.
The parietal stalk, or peduncle, emerges from the lateral surface of the
thalamus, and runs through the posterior limb of the internal capsule in close
association with the great motor tracts (Figs. 193, 196). It connects the lateral
nucleus of the thalamus with the cortex of the parietal and posterior part of the
frontal lobe.
Many of these fibers, especially those terminating in the posterior central
264 THE NERVOUS SYSTEM
gyrus, are afferent fibers of the third order mediating sensations of touch, heat,
cold, and perhaps also pain as well as sensations from the muscles, joints, and
tendons (Head, 1918). These sensory fibers are located behind the corticospinal
tract in the posterior limb of the internal capsule. According to Wilson (1914)
the medullary laminae of the lentiform nucleus do not contain any thalamocor-
tical fibers.
The occipital stalk, or peduncle, is also known as the optic radiation and as
the radiatio occipitothalamica. Its fibers stream out of the pulvinar and lateral
geniculate body, pass through the retrolenticular part of the internal capsule,
and run in a curved course toward the occiput, around the lateral side of the
posterior horn of the lateral ventricle to the cortex of the occipital lobe, and es-
pecially to the region of the calcarine fissure (Figs. 190, 191). It also contains
some fibers arising in the occipital cortex and ending in the superior quadrigeminal
body. We have learned that it forms an important part of the visual path
(Fig. 162).
Closely associated with the optic radiation in the retrolenticular part of the
internal capsule is the acoustic radiation (radiatio thalamotemporalis) . This
connects the medial geniculate body with the anterior transverse temporal gyrus
and the adjacent part of the superior temporal gyrus, and mediates auditory
sensations. It should be included as a part of the thalamic radiation.
CHAPTER XVII
THE RHINENCEPHALON
THE olfactory portions of the cerebral hemisphere may all be grouped to-
gether under the name rhinencephalon. Phylogenetically very old, this part of
the brain varies greatly in relative importance in the different classes of verte-
brates. The central connections of the olfactory nerves form all or almost all of
the cerebral hemispheres in the selachian brain (Fig. 13); while in the mammal
the non-olfactory cortex or neopallium has become the dominant part. Even
among the mammals there is great variation in the importance and relative
size of the olfactory apparatus. The rodents, for example, depend to a great
extent on the sense of smell in their search for food, and possess a highly developed
rhinencephalon. Such mammals are classed as macrosmatic. Man, on the
other hand, belongs in this respect with the microsmatic mammals, because in
his activities the sense of smell has ceased to play a very important part, and
his olfactory centers have undergone retrogressive changes. The carnivora and
ruminants are in an intermediate group. The sheep's brain furnishes a good
illustration of this intermediate type, and displays much more clearly than the
human brain the various parts of the rhinencephalon and their relation to each
other.
Parts Seen on the Basal Surface of the Brain. A comparison of the basal
surface of the sheep's brain with that of the human fetus of the fifth month shows
a remarkable similarity in the parts under consideration (Figs. 197, 198). The
olfactory bulb, which is the olfactory center of the first order, is oval in shape and
attached to the hemisphere rostral to the anterior perforated substance. It
lies between the orbital surface of the cerebral hemisphere and the cribriform
plate of the ethmoid bone. Through the openings in this plate numerous fine
filaments, the olfactory nerves, reach the bulb from the olfactory mucous mem-
brane. It contains a cavity, the rhinoccele, continuous with the lateral ventricle
(Fig. 182). In the adult human brain the cavity is obliterated and the connec-
tion between bulb and hemisphere is drawn out into the long olfactory tract.
This is lodged in the olfactory sulcus on the orbital surface of the frontal lobe
and in transverse section presents a triangular outline (Fig. 172). It contains
265
266
THE NERVOUS SYSTEM
olfactory fibers of the second order connecting the bulb with the secondary ol-
factory centers in the hemisphere. At its point of insertion into the hemisphere
the olfactory tract forms a triangular enlargement, the olfactory trigone.
From the point of insertion of the olfactory bulb or tract a band of gray
matter, the medial olfactory gyrus, can be seen extending toward the medial
surf ace of the hemisphere (Figs. 159, 197, 198). A similar gray band, the lateral
olfactory gyrus, runs caudalward on the basal surface of the sheep's brain. Along
Longitudinal fissure of cerebrum^
Optic nerve
Optic chiasma
Rhinal fissure
Insula
Lateral fissure-
Optic tract -
Infundibulum -
Mammillary body -.
Cerebral peduncle
Inter peduncular fossa and
nucleus
Trigeminal nerve
A bducens nerve
Acoustici Vestibular n -
nerve \Cochlearn. -
Glossopharyngeal nerve ~-~
Vagus nerve'
Hypo glossal nerve---'
Anterior median fissure
^V' Olfactory bulb
\ \' Medial olfactory gyrus
^^t, V
Anterior perforated substance
'Lateral olfactory stria
--'Lateral olfactory gyrus
^-Diagonal band
Amygdaloid nucleus
i ~ Pyriform area
-- Trochlear nerve
-~ Abducens nerve
~ Facial nerve
---- Trapezoid body
Cerebellum
- -Olive
^Chorioid plexus
" Accessory nerve
* Tractus later alls minor
Fig. 197. Ventral view of the sheep's brain.
its lateral border it is separated from the neopallium by the rhinal fissure; while
its medial border contains a band of fibers, the stria olfactoria lateralis (Fig. 197).
The same gyrus is seen in the brain of the human fetus, but here it is directed
outward toward the insula (Fig. 198). In the adult human brain these olfactory
convolutions are very inconspicuous, and with the fibers from the olfactory tract
which accompany them are usually designated as the medial and lateral olfactory
s tries.
THE RHINENCEPHALON
267
The medial olfactory gyrus and stria require further investigation. It has been gen-
erally supposed that the stria is formed by olfactory fibers of the second and third order
running to the olfactory centers in the rostral part of the medial surface of the hemisphere.
These are certainly few in number in the higher mammals, and Cajal (1911), who worked
chiefly with rodents, has been unable to identify any such fibers in these animals. The sig-
nificance of the medial olfactory gyrus is also obscure. According to Elliot Smith (1915),
"the rudiment of the hippocampal formation that develops on the medial surface begins
in front alongside the place where the stalk of the olfactory peduncle (which becomes the
trigonum olfactorium) is inserted; it passes upward to the superior end of the lamina termi-
nalis, from the rest of which it is separated by a triangular mass of gray matter called the
corpus paraterminale" (Fig. 200). This description, as well as the figure which accompanies
it, suggests a close relation between the rostral end of the hippocampal rudiment and what
is ordinarily known as the medial olfactory gyrus. The subdivision of the olfactory lobe
into anterior and posterior portions by the morphologically unimportant sulcus parol-
factorius posterior, although adopted in the B. N. A., is without justification and leads only
to confusion (Elliot Smith, 1907).
Olfactory bulb
Lateral olfactory gyrus (stria)
Posterior parolfactory sulcus
Amygdaloid nucleus
Medial olfactory gyrus (stria)
Olfactory tract
Limen insula
A nterior perforated substance
Hippocampal gyrus
Fig. 198. Brain of a human fetus of 22.5 cm. Ventral view. (Retzius, Jackson-Morris.)
Between the olfactory trigone and the medial olfactory gyrus, on the one
hand, and the optic tract on the other, is a depressed area of gray matter known
as the anterior perforated substance, through the openings in which numerous
small arteries reach the basal ganglia (Figs. 172, 197). The part immediately
rostral to the optic tract forms a band of lighter color, known as the diagonal
gyrus of the rhinencephalon or the diagonal band of Broca (Fig. 197). This
can be followed on to the medial surface of the hemisphere, where it is continued
as the paraterminal body or subcallosal gyrus (Fig. 200). Rostral to this gyrus
the hippocampal rudiment, which corresponds in part to the parolfactory area
of Broca, extends as a narrow band from the rostrum of the corpus callosum
toward the medial olfactory gyrus. In those mammals which possess an espe-
cially rich innervation of the nose and mouth, the region of the anterior per-
forated space is marked by a swelling, sometimes of considerable size, called
268
THE NERVOUS SYSTEM
the tuber culum olfactorium. According to Retzius, a small oval mass is present
in the anterior perforated substance of man immediately adjacent to the ol-
factory trigone, which represents this tubercle.
Olfactory bulb
Anterior commissure
Anterior perforated substance
-Amygdaloid nucleus
Pyriform area
Fig. 199. Ventral view of a sheep's brain, pyriform area shaded and anterior commissure
exposed.
The Pyriform Area. The lateral olfactory gyrus is continuous at its caudal
extremity with the hippocampal gyrus (Figs. 197, 198), and the two together
form the pyriform area or lobe (Fig. 199). In the adult human brain it is more
difficult to demonstrate the continuity of these parts. As the temporal lobe is
Hippocampal rudiment
Corpus callosum -.
Septum pellucidum
Fornix .
Anterior commissure.
Paraterminal body ?^~ , -=
Hippocampal rudiment -..
Olfactory trigone , *".,
Olfactory tract ^^^^
Olfactory bulb^
Intermediate olfactory stria''
Lateral olfactory gyrus and stria'
t
Anterior perforated substance
Limen insulce
L Hippocampus (gyri
Andrea Retzii)
-Fascia dentata
|T"-- Fimbria of hippocampus
" Hippocampus (proper)
Hippocampus
** Hippocampal gyrus
N - Cauda fascia dentata
Uncus
Diagonal band
Fig. 200. Diagram of the rhinencephalon.
thrust rostrally and the insula becomes depressed, the pyriform area is bent
on itself like a V (Fig. 198). The knee-like bend forms the limen insulce, and
with the rest of the insula becomes buried at the bottom of the lateral fissure.
The continuity of the pyriform area is not interrupted in the adult, though part
THE RHINENCEPHALON
269
of it is hidden from view. It includes the lateral olfactory stria and the cortex
subjacent to it (or lateral olfactory gyrus), the limen insulce, the uncus, and at
least a part of the hippocampal gyrus (Figs. 169, 172, 200). It is not easy to
determine just how much of the human hippocampal gyrus should be included.
Cajal (1911) apparently includes the entire gyrus, while Elliot Smith (1915)
limits it to the part of the gyrus dorsal to the rhinal fissure. In Fig. 200 we
have followed the outlines of the hippocampal region as given by Brodmann
(1909).
The Hippocampus. An olfactory center of still higher order is represented
by the hippocampus, which was seen in connection with the study of the lateral
Inferior horn of lateral ventricle
Hippocampus
Collateral eminence
Tapelum
Collateral trigone
Posterior horn of lateral ventricle
Hippocampal dictations
,' Uncus
Dentate fascia of hippocampus
Hippocampal gyrus
Hippocampal fissure
Fimbria of hippocampus
Bulb of posterior horn
Calcarine fissure
Calcar avis
Fig. 201. Part of temporal lobe of human brain showing inferior horn of lateral ventricle and the
hippocampus. Dorsal view. (Sobotta-McMurrich.)
ventricle. If we turn again to the floor of the inferior horn of the lateral ven-
tricle we shall see a long curved elevation projecting into the cavity (Figs. 181,
201). This is the hippocampus and is formed by highly specialized cortex
which has been rolled into the ventricle along the line of the hippocampal fissure
(Figs. 204, 209). It is covered on its ventricular surface by a thin coating of
white matter, called the alveus, which is continuous along its medial edge with
a band of fibers known as the fimbria of the hippocampus. This, in turn, is
continuous with the fornix (Fig. 201). In Figs. 201 and 204 there may be seen,
along the border of the fimbria, a narrow serrated band of gray matter, the
fascia dentata, which lies upon the medial side of the hippocampus. It is sepa-
rated from the hippocampal gyrus by a shallow groove, called the hippocampal
270 THE NERVOUS SYSTEM
fissure, that marks the line along which the hippocampus has been rolled into
the ventricle.
The hippocampus and fascia dentata belong to the archipallium. In the
marsupials and monotremes this extends dorsally on the medial surface of the
hemisphere in a curve, which suggests that of the corpus callosum (Fig. 202).
In the higher mammals the presence of a massive corpus callosum seems to
inhibit the development of the adjacent part of the hippocampal formation,
which remains as the vestigial indusium griseum, or supracallosal gyrus. This
hippocampal rudiment is a thin layer of gray matter on the dorsal surface of the
corpus callosum, within which are found delicate strands of longitudinal fibers.
Two of these strands, placed close together on either side of the median plane,
Cerebral cortex
^.
, Hippocampal fissure
Hippocampus and fascia
dentata
Chorioid fissure
- Thalamus
Olfactory bulb . ^_^- , _,
^ ^ " Pynform area
Tuberculum olfactorium
Rhinal fissure
Fig. 202. Median view of the cerebral hemisphere of a monotreme Ornithorhynchus. (Elliot
Smith.)
are more conspicuous than the others, and are known as the medial longitudinal
stria. On either side, where the supracallosal gyrus bounds the sulcus of the
corpus callosum, there is a less distinct strand, the lateral longitudinal stria
(Figs. 174, 175). The hippocampal rudiment can be traced upon the medial
surface of the hemisphere from the region of the medial olfactory gyrus (or stria)
toward the rostrum of the corpus callosum, then around the dorsal surface of
that great commissure to the splenium, behind which' it becomes continuous
with the hippocampus proper, where this comes to the surface in the angle
between the fascia dentata and the hippocampal gyrus (Fig. 200 Elliot Smith,
1915).
The Fornix. Within the hippocampus fibers arise which run through the
white coat on its ventricular surface, known as the alveus, into thefimbria. This
THE RHINENCEPHALON 271
is a thin band of fibers, running along the medial surface of the hippocampus
and joining with the alveus to form the floor of the inferior horn of the lateral
ventricle (Figs. 201, 204, 209). The fimbria increases in volume as it is traced
toward the splenium of the corpus callosum, to the under surface of which it
becomes applied, where, together with its fellow of the opposite side, it forms
the fornix.
The fornix, which is represented diagrammatically in Fig. 203, is an arched
fiber tract, consisting of two symmetric lateral halves, which are separate at
either extremity, but joined together beneath the corpus callosum. This
medially placed portion is known as the body of the fornix. From its caudal
extremity the fimbria diverge, and one of them runs along the medial aspect of
each hippocampus. In man the hippocampus does not reach the under surface
Column of fornix
Body of fornix
- Hippocampal commissure
|
Cms of fornix
Fimbria of hippocampus
Fig. 203. Diagram of the fornix.
of the corpus callosum, and the part of the fimbria which joins the body of the
fornix, being unaccompanied by hippocampus, is known as the cms fornicis.
Rostrally the fornix is continued as two arched pillars, the columnce fornicis,
to the mammillary bodies.
The body of the fornix is triangular, with its apex directed rostrally. It con-
sists in large part of two longitudinal bundles of fibers, representing the con-
tinuation of the fimbriae, widely separated at the base of the triangle, but closely
approximated at the apex, whence they are continued as the columnae fornicis.
At the point where these longitudinal bundles diverge toward the base of the
triangle they are united by transverse fibers which join together the two hippo-
campi by way of the fimbriae. These fibers constitute the hippocampal com-
missure. This part of the fornix, because of its resemblance to a harp, was
formerly known as the psalterium (Fig. 184). The hippocampal commissure
272
THE NERVOUS SYSTEM
is not very evident in the human brain, but can be easily dissected out in the
sheep (Fig. 204).
The columns fornicis are round fascicles which can be traced ventrally in
an arched course to the mammillary bodies (Figs. 203-205). They are placed
on either side of the median plane. Each consists of an initial free portion,
which forms the rostral boundary of the interventricular foramen, and a cov-
ered part, which runs through the gray matter in the lateral wall of the third
ventricle to reach the mammillary body (Figs. 204, 205).
The relations of the fornix are well shown in Figs. 155, 200, and 205. The
body of the fornix intervenes between the corpus callosum, septum pellucidum,
Body of corpus callosum
Lateral ventricle
Genu of corpus callosum
Body of fornix
Hippocampal commissure
! Thalamus
Splenium of corpus callosum
. Lateral ventricle
Chorioid fissure
Hippocampus
Anterior commissure
- Fimbria of hippo-
campus
-W- Hippocampal
fissure
-/- Hippocampal gyms
^^^r^
' Dentate fascia
Mammillothalamic tract
Mammillary body
Infundibulum
Fig. 204. Dissection of the cerebral hemisphere of the sheep to show the fornix and hippocampus.
Median view.
Lamina terminalis
Optic chiasma \
Column of fornix.
and cavity of the lateral ventricle on the one hand, and the transverse fissure of
the cerebrum and the thalamus on the other. The fimbria and body of the for-
nix form one boundary of the chorioid fissure. This fissure, which is shown but
not labeled in Fig. 205, represents the line along which the chorioid plexus is
invaginated into the lateral ventricle. When this plexus has been torn out,
the fissure communicates with the interventricular foramen.
The septum pellucidum is the thin wall which separates the two lateral ven-
tricles and fills in the triangular interval between the fornix and the corpus
callosum (Fig. 205). It consists of two thin vertical laminae separated by a
cleft-like interval, the cavity of the septum pellucidum (Fig. 177). Each lamina
THE RHINENCEPHALON
273
forms part of the medial wall of the corresponding hemisphere (Fig. 182); and
the cavity, although sometimes called the fifth ventricle, develops as a cleft
within the lamina terminalis and, therefore, bears no relation to the true brain
ventricles, which are expansions of the original lumen of the neural tube (Fig.
165).
The anterior commissure, like the hippocampal commissure, belongs to the
rhinencephalon. It is a rounded fascicle which crosses the median plane in the
dorsal part of the lamina terminalis just rostral to the columnae fornicus (Fig.
205). In a frontal section of the brain, like that represented in Fig. 187, it can
Splenium of corpus callosum
Sulcus cinguli
Parieto-occipital fissure
Cuneus
Cakarine \
fissure
,/ // Body of corpus callosum
Body of form* / " Free potion of col. of fornix
Septum pdlucidum
Intervent. foramen
Anterior commiss.
Genu of
corpus
callosum
Occipila
lobe
Crus offornix
Thalamus
Fimbria of hippocampus
Dentate fascia of hippocampus
Uncus
Olfactory
bulb
Olfactory tract
Rostrum offorpus col.
, Rostral lamina
Optic nerve
' 'Covered portion of column of
Mammillary body fornix
Mammillothalamic tract
Fig. 205. Dissection of the human cerebral hemisphere to show the fornix. Median view.
(Sobotta-McMurrich.)
be traced lateralward through the most ventral part of the lentiform nucleus.
It consists of two parts (Fig. 206). Of these, the more rostral is shaped like a
horseshoe and joins together the two olfactory bulbs. This part can be readily
dissected out in the sheep's brain (Fig. 199), but is poorly developed in man. The
remaining portion, and in man the chief component, joins the pyriform areas
of the two hemispheres together (Cajal, 1911).
We are now sufficiently acquainted with the anatomy of the rhinencephalon
to undertake a study of the structure and connections of its various parts.
Because of the wealth of detail which this subject offers we must confine our at-
18
274
THE NERVOUS SYSTEM
tention to the more important facts. Cajal (1911) has carried out extensive
investigations concerning the structure and connections of the olfactory parts
of the brain both in man and the smaller macrosmatic mammals, especially the
mouse. His results, which differ in many respects from the ideas previously
current, have been brought together in his "Histologie du Systeme Nerveux,"
Vol. II, pp. 646-823. The account which follows is largely based on his work.
Fig. 206. Horizontal section of the rostral portion of the cerebral hemispheres of a mouse to
show the anterior commissure. Golgi method. A, anterior and B, posterior portions of anterior
commissure; G, anterior column of the fornix. (Cajal.)
Structure and Connections of the Olfactory Bulb. In the olfactory portion
of the nasal mucous membrane there are located bipolar sensory cells, each with a
thick peripheral process, the ciliated extremity of which reaches the surface of
the epithelium. These are the olfactory neurons of the first order, and their
slender central processes are the unmyelinated axons which constitute the olfac-
tory nerves. These fibers are gathered into numerous small bundles, the fila-
THE RHINENCEPHALON
275
ments of the olfactory nerve, which pass through the cribriform plate of the eth-
moid bone and immediately enter the olfactory bulb (Fig. 207). Here they
form a feltwork of interlacing fibers over that surface of the bulb which is in
contact with the cribriform plate.
The olfactory bulb of man is solid, and the original cavity is represented by a
central gray mass of neuroglia. This is surrounded by a deep layer of myelinated
Fig. 207. Diagram showing the direction of conduction in the olfactory nerve bulb and tract:
A, lateral olfactory stria; B, anterior portion of the anterior commissure; C, bipolar cells of the
olfactory mucous membrane. (Cajal.)
nerve-fibers passing to and from the olfactory tract. Superficial to this are several
layers of gray matter of very characteristic structure, and this, in turn, is covered
with the superficial layer of unmyelinated fibers from the olfactory nerve fila-
ments. Within the gray matter of the bulb are found three types of neurons,
the mitral, tufted, and granule cells. The large mitral cells are the most char-
276
THE NERVOUS SYSTEM
acteristic element; and their perikarya are closely grouped together, forming
a well-defined layer (Fig. 208, C). The tufted cells are smaller and more super-
ficially placed (Fig. 208, B). The larger dendrites from both these types of'
neurons are directed toward the superficial fiber layer. Each of these dendrites
Fig. 208. Section of the olfactory bulb of a kitten. Golgi method. A, Layer of glomeruli;
B, external plexiform layer; C, layer of mitral cells; D, internal plexiform layer; E, layer of granules
and white substance; /, J, granule cells; a, b, glomeruli, showing the terminations of the olfactory
nerve-fibers; c, glomerulus, showing the terminal arborization of a dendite of a mitral cell; d,
tufted cells; e, mitral cell; h, recurrent collateral from an axon of a mitral cell. (Cajal.)
breaks up into many branches, which form a compact rounded bushy terminal.
The terminal ramifications of olfactory nerve-fibers interlace with these dendritic
branches, and the two together form a circumscribed, more or less spheric ol-
factory glomerulus (Fig. 208, A). These relations were demonstrated by Cajal
THE RHINENCEPHALON 277
in 1890, and possess considerable theoretic and historic interest. Since in these
glomeruli the olfactory nerve-fibers come into contact with only the dendritic
ramifications of the mitral and tufted cells, it is evident that these dendrites
must take up and transmit the olfactory impulses. That is to say, these glomer-
uli furnished positive proof that the dendrites are not, as had been thought by
many investigators, merely root-like branches which serve for the nutrition of
the cell. The mitral cells are larger than the tufted cells and their axons are
thicker. These coarse axons are directed for the most part into the lateral ol-
factory stria; while the finer axons of the tufted cells pass through the anterior
commissure to the opposite olfactory bulb (Fig. 207). The axons of the deeply
placed granule cells are relatively short and are directed toward the surface of
the bulb.
The olfactory tract consists of fibers passing to and from the olfactory bulb.
Through it each bulb receives fibers from the other by way of the anterior com-
missure as well as from the hippocampal cortex. The fibers leaving the olfac-
tory bulb are the axons of the mitral and tufted cells. By far the greater number
of the axons of the mitral cells are continued into the lateral olfactory stria. A
much smaller number terminates in the olfactory trigone and in the tuberculum
olfactorium within the anterior perforated substance. Other fibers are said to
pass by way of the medial olfactory stria to the parolfactory area of Broca, to
the subcallosal gyrus, and to the septum pellucidium, but this is open to ques-
tion. The fibers of the lateral olfactory stria run upon the surface of the lateral
olfactory gyrus, also known as the frontal olfactory cortex, to which they give
off collaterals (Fig. 207). The terminal fibers reach the uncus and part of the
hippocampal gyrus. The chief olfactory centers of the second order are, there-
fore, found in the pyriform area.
According to Cajal (1911), the hippocampal gyrus may be subdivided in man, as in the
mammals, into five areas: (1) the external region near the rhinal fissure; (2) the principal
olfactory region, the most salient part of the convolution; (3) the presubiculum, a transitional
area between 2 and 4; (4) the subiculum, near the hippocampal fissure, and (5) the caudal
olfactory region, including the caudal part of the hippocampal gyrus. Of these five regions,
Cajal finds fibers from the lateral olfactory stria going to the second or principal olfactory
region only. The presubiculum and subiculum and the caudal olfactory region represent
olfactory association centers. The subiculum is characterized by the presence of a thick
layer of myelinated fibers upon its surface.
The hippocampus, which constitutes an olfactory center of a still higher
order, is directly continuous with the portion of the hippocampal gyrus known
as the subiculum (Fig. 209), and is formed by a primitive portion of the cortex
278
THE NERVOUS SYSTEM
that has been rolled into the ventricle along the line of the hippocampal fissure.
Upon its ventricular surface it is covered by a thin layer of white matter, known
as the alveus, through which the fibers arising in the hippocampus reach the
fimbria and the fornix. Beginning at the line of separation from the fascia
dentata, we may enumerate the constituent layers of the hippocampus as fol-
lows: the molecular layer, the layer of pyramidal cells, and the layer of poly-
morphic cells (Figs. 209, 210).
The molecular layer contains a superficial stratum of tangential fibers derived
from the corresponding layer of the subiculum and from bundles of fibers that
Fig. 209. Cross-section of the hippocampus and hippocampal gyrus of man. (Edinger.)
perforate the cortex of the subiculum (Fig. 210). More deeply placed is another
fiber layer, containing collaterals from the pyramidal cells as well as collateral and
terminal fibers from the alveus, and known as the stratum lacunosum. The
molecular stratum in the hippocampus resembles that in other parts of the cortex
in containing the terminal branches of the apical dendrites from the pyramidal
cells, and a few nerve-cells which for the most part belong to Golgi's Type II.
The Layer of Pyramidal Cells. The pyramidal cells are all of medium size
and their fusiform bodies are rather closely packed together, forming a well-
THE RHINENCEPHALON
279
defined zone, the stratum lucidum. Their apical dendrites are directed toward
the molecular layer and form the chief constituent of the stratum radiatum.
The axons of these cells, after giving off collaterals, enter the alveus.
The layer of polymorphic cells, also known as the stratum oriens, contains
cells of Martinotti, that send their axons into the molecular layer, and still other
cells the axons of which enter the alveus.
The alveus is a thin white stratum which separates the preceding layer from
the ventricle. It is continuous, on the one hand, with the white center of the
Alveus
Layer of polymorphic cells
Layer of pyramidal
cells
Stratum lucidum\
Stratum radia-
tum
. Molecular layer
I Stratum lacunosum
Tangential fibers
Lateral ventricle
Fimbria
Hippocampus /
Fascia dentata
Molecular layer
Granule layer
Layer of polymorphic cells
Subictilum
Fig. 210. Diagram of the structure and connections of the hippocampus. The arrows
show the direction of conduction: A, molecular layer, and B, pyramidal cell layer of the subic-
ulum; F, hippocampal fissure. (Cajal.)
hippocampal gyrus, and on the other with the fimbria. Through it the efferent
fibers of the hippocampus enter the fimbria and fornix. The fibers of the hippo-
campal commissure are also carried in the fimbria and enter the hippocampus
through the alveus.
The fascia dentata also belongs to the archipallium and is closely related to
the hippocampus, which it resembles somewhat in the structure of its three
strata: the molecular layer, granule layer, and layer of polymorphic cells (Fig.
210). The granules may be regarded as modified pyramidal cells of small size,
ovoid or fusiform in shape. Each possesses instead of a single apical dendrite
'two or three branching processes which extend into the molecular layer. The
2 g THE NERVOUS SYSTEM
axons are directed into the layer of pyramidal cells of the hippocampus. Orig-
inally this layer of pyramidal cells was continuous with the granule layer of
the fascia dentata, but in all the higher mammals a break in this cellular stratum
has occurred at the point of transition between the two divisions of the archi-
pallium.
THE OLFACTORY PATHWAYS
Impulses reach the glomeruli of the olfactory bulb along the fibers of the
olfactory nerve and are here transferred to the dendrites of the mitral cells.
Axons arising from these cells and running in the lateral olfactory stria transmit
the impulses to the pyriform area (Fig. 207), whence they are conveyed to the
hippocampus and fascia dentata by fibers entering the molecular layer in both
of these parts of the hippocampal formation (Fig. 210).
According to Cajal, the fibers of the lateral olfactory stria terminate in the principal
olfactory region of the hippocampal gyrus, and there are present within the cortex of the
pyriform area sagittal association fibers which unite the principal olfactory region with the
caudal olfactory region of the hippocampal gyrus. From this latter region fibers reach the
hippocampus and fascia dentata. These are relatively thick fibers which are found at first
in the angle of the subiculum and can be traced through all the layers of that center into
the molecular layer of the hippocampus and fascia dentata (Fig. 210, B). Within the molec-
ular layer the impulses are transferred from these fibers to the dendrites of the pyramidal
and granule cells. It was formerly supposed that fibers from the trigonum olfactorium,
substantia perforata anterior, and septum pellucidum reached the hippocampus through
the strize longitudinales and the fornix, and served as the chief conductors of afferent im-
pulses toward the hippocampus. But according to Cajal, "The hippocampus does not receive
olfactory impulses from the frontal region of the brain, nor through the intermediation of the
septum pellucidum."
The efferent fibers from the hippocampus represent the axons of the pyra-
midal cells. These penetrate the stratum oriens and enter the alveus (Fig.
210). Thence they are continued into the fimbria and fornix. They include
both commissural and projection fibers. The commissural fibers serve to unite
the two hippocampi and run through the hippocampal commissure as the trans-
verse fibers of the psalterium. The projection fibers are continued rostrally;
and in their course through the body of the fornix they form on either side of
the median plane a longitudinal bundle, which is continued into the columna
fornicis (Fig. 203). The latter bends caudally into the hypothalamic region,
giving off fibers to the tuber cinereum and the mammillary body. The remaining
fibers of the columna fornicis undergo a decussation just behind the mamillary
body and are continued in the reticular formation of the brain stem as far, at
least, as the pons. It will be obvious that the fornix is the efferent projection
THE RHINENCEPHALON
28l
tract of the archipallium and serves to convey impulses from the hippocampus
to the hypothalamus and reticular formation of the brain stem. Through the
mammillary bodies olfactory impulses are relayed along the mammillothalamic
tract to the anterior nucleus of the thalamus, and along the mammillotegmental
bundle to the tegmentum of the pons and medulla oblongata (Fig. 21 1,/, g).
The frontal olfactory projection tract takes origin from the gray matter of
the olfactory peduncle or trigonum olfactorium and the gyrus olfactorius later-
Fig. 211. Diagram of the afferent and efferent paths of the mammillary body, habenular
ganglion, and interpeduncular ganglion: A, Medial nucleus of the mammillary body; B, C,
anterior nucleus of the thalamus; D, habenular ganglion; E, interpeduncular ganglion; F, dorsal
tegmental nucleus; J, optic chiasma; T, tuber cinereum; P, pons; a, cerebral aqueduct; b, habenular
commissure; c, posterior commissure; d, fasciculus retroflexus of Meynert; e, peduncle of the mam-
millary body;/, fasciculus mamillothalamicus; g, tegmental tract of Gudden; h, frontal olfactory
projection tract; i, stria medullaris thalami. The arrows indicate the direction of conduction.
(Cajal.)
alis. It traverses the subthalamic region to reach the pons and medulla oblon-
gata. A bundle of fibers, consisting in part of collaterals, is given off from it,
to enter the stria medullaris thalami, which we have already traced to the habe-
nular ganglion (Fig. 211, h,i).
The stria terminalis is a delicate fascicle of nerve-fibers which lies in the sulcus between
the thalamus and caudate nucleus (Figs. 155, 177), and accompanies the tail of the latter in
282 THE NERVOUS SYSTEM
the roof of the inferior horn of the lateral ventricle. According to Cajal (1911), it contains
both commissural and projection fibers, the majority of which take origin from the olfactory
cortex of the hippocampal gyms. A smaller number may arise in the amygdaloid nucleus.
After following the curved course of the caudate nucleus, it bends ventrad toward the
anterior commissure. Some of the fibers cross in the anterior commissure and end in the
olfactory cortex of the opposite temporal lobe and in the septum pellucidum. The majority
of the fibers, however, enter the mesencephalon and apparently end in the interstitial nucleus.
The striae longitudinales, fornix longus, and the fiber tracts found in the
subcallosal cortex and septum pellucidum have apparently been subject to
much misinterpretation; but the subject is too extensive to be considered here.
(See Cajal, Histologie du Systeme Nerveux, Vol. II, pp. 783-823.)
The anterior perforated substance, or at least its more rostral part, which
corresponds to the tuberculum olfactorium of macrosmatic mammals, receives
besides fibers from the olfactory tract other afferent fibers which, according to
Edinger (1911), come from the pons, perhaps from the sensory nucleus of the
trigeminal nerve. It is probably "especially concerned with the feeding reflexes
of the snout or muzzle, including smell, touch, taste, and muscular sensibility,
a physiologic complex which Edinger has called collectively the 'oral sense' "
(Herrick, 1918).
CHAPTER XVIII
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL
HEMISPHERE
THE cerebral cortex forms a convoluted gray lamina, covering the cerebral
hemisphere, and varies in thickness from 4 mm. in the anterior central gyrus
to 1.25 mm. near the occipital pole. When sections through a fresh brain are
examined macroscopically, the cortex is seen to be composed of alternating
lighter and darker bands, the light stripes being produced by aggregations of
myelinated nerve-fibers (Fig. 212).
Nerve-fibers. In addition to a very thin superficial white layer of tangential
fibers there are in most parts of the cerebral cortex two well-defined white bands,
the inner and outer lines of Baillarger
(Figs. 212, 215). These two bands con-
tain large numbers of myelinated nerve-
fibers running in planes parallel to the
surface of the cortex. In the region of
the calcarine fissure only the outer line
is visible; but this is very conspicuous
and is here known as the line of Gennari.
Myelinated fibers enter the cortex from
the white center in bundles that in
general have a direction perpendicular
to the surface of the cortex. These
bundles radiate into each convolution from its central white core and separate
the nerve-cells into columnar groups, thus giving the cortex a radial striation
(Fig. 215).
Many of the fibers in these radial bundles are corticifugal, representing the
axons of the pyramidal and polymorphic cells of the cortex. Within the medul-
lary center they run (1) as association fibers to other parts of the cortex of the
same hemisphere, (2) as commissural fibers through the corpus callosum to the
opposite hemisphere, or (3) as projection fibers to the thalamus and lower .lying
centers. The others are corticipetal and are derived in part from the thalamic
radiation; but an even greater number of them are the terminal portions of as-
283
Fig. 212. Schematic sections of cerebral
gyri showing the alternate lighter and darker
bands which compose the cerebral cortex: 1
shows the layers as seen in most parts of the
cerebral cortex; 2, the layers as seen in the
region of the calcarine fissure. (Baillarger,
Quain's Anatomy.)
284
THE NERVOUS SYSTEM
sociation and commissural fibers from other parts of the cortex. Many of these
fibers end in the most superficial stratum of the cortex, the plexiform layer, where
the terminal branches of the apical dendrites of the pyramidal cells are widely
expanded (Fig. 214). Others terminate as indicated in Fig. 213, where they
Fig. 213. From the anterior central gyrus of
the human cerebral cortex, showing the terminations
of corticipetal fibers: a, b, Afferent fibers; B, dense
network produced by the terminal branches of such
fibers. Golgi method. (Cajal.)
Fig. 214. Nerve-cells and neuroglia
from the cerebral cortex: A, Neuroglia; B,
horizontal cells of Cajal ; C, pyramidal cells;
D, cell of Martinotti; E, stellate cell.
are seen forming a close network of unmyelinated fibers. Enmeshed in the
dense fiber plexus indicated at B, Fig. 213, are the pyramidal cells illustrated
in Layer III of Fig. 215.
The nerve-cells of the cortex are disposed in fairly definite layers as indicated
in Fig. 215. We may enumerate five well-recognized varieties: (1) the pyra-
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 285
midal, (2) the stellate, and (3) the polymorphous cells, as well as (4) the hori-
zontal cells of Cajal, and (5) the cells of Martinotti.
The pyramidal cells are the most numerous and are classified as small,
medium, large, and giant pyramidal cells (Fig. 215). From the base of a pyra-
midal cell body an axon extends toward the subjacent white matter, giving
off collaterals which ramify in the adjacent cortex (Figs. 23, 214, C). The den-
drites are of two kinds: a large apical dendrite and numerous smaller ones at-
tached to the base and sides of the pyramid. The apical dendrite appears as an
extension of the cell body and is directed toward the surface of the cortex, near
which it ends in spreading branches. Its length varies with the depth of the
cell body from the surface. To an even greater extent than other dendrites it is
provided with short thorny processes called "spines" or "gemmules." These
are supposed by some to effect contact with neighboring axonic ramifications
and to be retractile. Upon retraction of these gemmules, conduction across
the synapse would be interrupted for the time being; and one might explain
the varying sensory thresholds of an individual in sleep or during attention by
the varying degree of expansion of the gemmules. But as yet no satisfactory
evidence in support of the theory has been presented.
The stellate cells are also known as granules. They are, for the most part,
of small size, and their short axons branch repeatedly and terminate in the
neighborhood of the cell of origin. That is to say, they are cells of Golgi's
Type II. Although they occur in most layers of the cortex, they are especially
numerous in the fourth stratum, which is accordingly designated as the layer
of small stellate cells (Figs. 214, ; 215).
The cells of Martinotti, which are also found in most of the cortical strata,
have this as their distinguishing characteristic, that their axons are directed
toward the surface of the cortex and ramify in the superficial layer (Fig. 214, D}.
The horizontal cells of Cajal, which are present only in the superficial layer,
are fusiform, with long branching dendrites directed horizontally. Their axons
are long and form tangential myelinated fibers in the superficial layer (Fig. 214,5).
Polymorphous cells, fusiform or angular in shape, are found in the deepest
stratum of the cortex (Figs. 214, 215). Their axons enter the subjacent white
matter.
CELL AND FIBER LAMINATION
The size and type of cells found in the cortex vary at different depths from
the surface, that is to say, the cells are disposed in fairly definite layers. As
already indicated, many of the myelinated fibers are arranged in bands parallel
286
THE NERVOUS SYSTEM
to the surface. By means of this cell and fiber lamination Brodmann (1909)
recognizes six layers in the cerebral cortex (Fig. 215). Other authors, notably
Campbell (1905) and Cajal (1906), number these layers somewhat differently.
Moreover, the arrangement varies in different parts of the cortex. In certain
Via
VIb
> - >-
4({\\$^
Fig. 215. Diagram of the structure of the cerebral cortex: 7, Molecular layer; II, layer of
small pyramidal cells; ///, layer of medium-sized and large pyramidal cells; IV, layer of small
stellate cells; V, deep layer of large pyramidal cells; VI, layer of polymorphic cells; ja 1 , band of
Bechterew; 4, outer band of Baillarger; 56, inner band of Baillarger. (Brodmann.)
regions one or more of the strata may be reduced, enlarged or subdivided, but
the arrangement in most parts is substantially like that illustrated. The six
layers are as follows:
1. The molecular layer (plexiform layer, lamina zonalis) is the most super-
ficial. It contains the superficial band of tangential myelinated fibers and many
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 287
neuroglia cells. The nerve-cells are of two kinds: (1) horizontal cells of Cajal,
and (2) cells of Golgi's Type II. Within this layer ramify the terminal branches
of the apical dendrites from the pyramidal cells of the deeper layers.
2. The layer of small pyramidal cells (lamina granularis externa) contains a
large number of small nerve-cells. Most of these are small pyramids with axons
running to the white center of the hemisphere. Others belong to the short-
axoned group (Golgi's Type II).
3. The layer of medium-sized and large pyramidal cells (lamina pyramidalis)
may be subdivided into two substrata, the more superficial stratum containing
chiefly medium-sized pyramids and the deeper one chiefly large pyramids. There
are also present cells of Golgi's Type II and cells of Martinotti. According to
Cajal (1900-1906) and Campbell (1905), it is within this layer that the outer
stripe of Baillarger is located, but Brodmann places this line in the next layer.
4. The layer of small stellate cells (lamina granularis interna) is characterized
by the presence of a large number of small multipolar cells with short axons
(Golgi's Type II). Scattered among these are small pyramids. Brodmann
places the outer line of Baillarger in this stratum.
5. The deep layer of large pyramidal cells (lamina ganglionaris) contains the
largest cells of the cortex. In the motor region these are known as the giant
pyramidal cells of Betz and give origin to the fibers of the corticospinal tract.
The apical dendrites of these cells are very long and, like those of the more super-
ficial pyramidal cells, reach and ramify within the molecular layer. Smaller
cells, both of the pyramidal and short-axoned type, are also present. The
horizontal fibers of Baillarger 's internal line are found in this layer in most of
the cortical areas.
6. The layer of polymorphic cells (lamina multiformis) contains irregular
fusiform and angular cells, the axons of which enter the subjacent white matter.
Cortical Areas. The six layers of the cortex are arranged in most regions
essentially as shown in Fig. 215. But each of more than forty areas presents its
own characteristic variation in the structure, thickness, and arrangement of
the cellular layers, in the thickness of the cortex as a whole, in the number of
afferent and efferent myelinated fibers, and in the number, distinctness, and posi-
tion of the white striae. On the basis of such differences the entire cortex has
been subdivided into structurally distinct areas. Maps of such cortical areas
have been furnished by Brodmann (1909), Campbell (1905), and Elliot Smith
(1907) ; and while these vary in detail, they agree in their larger outlines. The
existence and general boundaries of these regions are now well established; and
288
THE NERVOUS SYSTEM
as a result of experimental and pathologic research it is known that specific
differences in function are correlated with these differences in structure.
The maps of the cortical areas furnished by Brodmann are reproduced in
Figs. 216 and 217. He recognizes eleven general regions, and each of these may
20
Fig. 217.
Figs. 216 and 217. Areas of the human cerebral cortex each of which possesses a distinctive
structure: Fig. 216, lateral view; Fig. 217, medial view. (Brodmann.)
be subdivided into smaller areas on the basis of characteristic differences in
structure. Some of these differences are visible to the naked eye and have
been represented in Fig. 218.
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 289
Myelination. The fibers in the various parts of the cortex acquire their
myelin sheaths at different tunes. On this basis Flechsig (1896) identified
thirty-six areas, which are numbered in Fig. 219 in the order of myelination. He
recognizes three main groups: primary (Nos. 1 to 12), intermediate (Nos. 13
Fig. 218. Diagram showing the differences in thickness and in the arrangement of the lighter
and darker bands in the human cerebral cortex in different regions as seen with the naked eye:
A, Motor cortex from anterior central gyrus; B, sensory cortex from the posterior central gyms;
C, visual cortex from the region of the calcarine fissure; D, auditory cortex from the anterior
transverse temporal gyrus. (Redrawn after Elliot Smith.)
to 28), and late (Nos. 28 to 36). According to Flechsig, the primary areas,
which are myelinated at birth, are projection centers and receive the sensory
radiation from the thalamus; while the other parts of the cortex, not being pro-
vided with projection fibers, serve only as association centers. He believed that
Fig. 219. Lateral view of the human cerebral hemisphere, showing the cortical areas as
outlined by Flechsig on the basis of differences in the time of myelination of their nerve-fibers.
The primary areas (first to become well myelinated) are cross-hatched; the intermediate are
indicated by vertical lines; the late areas are unshaded. (Lewandowsky.)
myelination of nerve-fibers takes place in the order of conduction, that is, the
sheaths are developed first on the afferent fibers, reaching the cortex from the
thalamus, and later on the association fibers, linking the various areas together.
According to this conception fibers of like function tend to become myelinated
19
2 QO THE NERVOUS SYSTEM
at the same time. Much of Flechsig's work has failed to stand the test of rigid
examination. It is now known that practically all regions of the cortex, in-
cluding those designated by him as association centers, are connected with the
thalamus or lower lying centers by afferent or efferent projection fibers. It
has been shown that the more mature areas fade off gradually into those whose
differentiation is less advanced, and that sharply outlined zones such as are
indicated in his figures do not exist. Nevertheless, it is true that the regions
designated by him as primary areas, though not sharply outlined by this method
from the surrounding cortex, do mature first, and the myelination spreading
from these areas reaches its completion last in those areas included in his late
group (Brodmann, 1910). The primary areas include the region surrounding
the central fissure, the region around the calcarine fissure, a portion of the
superior temporal gyrus, and a part of the hippocampal gyrus. These areas
are associated with especially important projection tracts and may properly
be spoken of as projection centers.
CORTICAL OR CEREBRAL LOCALIZATION
In opposition to the crude conceptions of the localization of cerebral functions
introduced by Gall (1825), which formed the basis for phrenology, the French
physiologist Florens maintained the doctrine that all parts of the cerebrum are
functionally equivalent. In 1861 Broca demonstrated that destruction of the
left third frontal convolution may result in a loss of ability to speak; and nine
years later Fritsch and Hitzig (1870) discovered that electric excitation of the
cortex in the region of the central sulcus will elicit movements from muscles of
the opposite side of the body. These observations, confirmed and extended
by many observers, definitely proved that certain cortical areas possess spe-
cialized functions. Physiologic and pathologic researches have served to out-
line a number of these with considerable precision, and it is possible to identify
them with regions of characteristic cell and fiber lamination. In this way evi-
dence derived from histologic studies reinforces that drawn from physiology and
pathology.
The motor projection center is located in the anterior -wall of the central sulcus,
in the adjacent part of the anterior central gyrus, and in that part of the para-
central lobule which lies rostral to the continuation of the central sulcus on the
medial surface of the hemisphere (Figs. 220, 221). It coincides fairly closely
with Area 4 of Brodmann's charts (Figs. 216, 217). This is the center from which
the impulses initiating voluntary movements on the opposite side of the body
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 2QI
descend to the motor nuclei of the cerebrospinal nerves. It is subdivided into
areas, each of which controls the muscles moving a given part of the opposite
half of the body; and these are arranged in inverted order, beginning with the
center for movement of the toes near the dorsal border of the hemisphere, and
ending with that for the face at the lower end of the anterior central gyrus (Fig.
236).
The structure of the motor cortex is characteristic. Here the gray matter
reaches the maximum thickness, the lines of Baillarger are broad and diffused
(Fig. 218). The fifth layer contains the giant pyramidal cells of Betz, from
which arise the fibers of the corticospinal and corticobulbar tracts. These
cells undergo chromatolysis when these motor tracts are cut; and when the motor
cortex is destroyed the tracts degenerate (Holmes and May, 1909).
Motor projection center
Somesthetic area
Auditory re-
ceptive center
Motor projection center
Somesthetic area
Visual receptive center
Fig. 220. Diagram of the cortical pro-
jection centers on the lateral aspect of the
cerebral hemisphere.
Visual re-
Olfactory center ce P the center
Fig. 221. Diagram of the cortical pro-
jection centers on the medial aspect of the
cerebral hemisphere.
The motor cortex of the chimpanzee corresponds in its arrangement with
that of man; and by the electric excitation of its different portions muscular
contractions can be excited in the corresponding parts of the opposite side of
the body (Griinbaum and Sherrington, 1903). In addition, there is an area
farther forward in the frontal lobe the stimulation of which produces conjugate
movements of the eyes. A similar center for the conjugate deviation of the
head and eyes is situated in the posterior part of the middle frontal gyrus in
man. It is probable, however, that this motor center is of a different kind
from those found in the anterior central gyrus, from which all of the fibers of
the pyramidal system are believed to take their origin (Fig. 236).
The sensory projection centers are the areas within which terminate the
sensory projection fibers. We have learned to locate such centers for vision,
THE NERVOUS SYSTEM
hearing, smell, and the general sensations from the surface of the body and the
deeper tissues. The latter region, known as the common sensory or somesthetic
area, is located in the posterior central gyrus (Areas 1, 2, and 3 of Brodmann).
It receives fibers belonging to the thalamic radiation from the lateral nucleus of
the thalamus and representing neurons of the third order in the afferent paths
from the skin, muscles, joints, and tendons.
The most conclusive evidence of the sensory function of the posterior central
gyrus is furnished by Cushing's (1909) observations on the electric excitability
of the human cerebral cortex. These tests were made on unanesthetized patients
in the course of operations for brain tumors. Stimulation of the cortex within
the posterior central gyrus caused the patients to experience cutaneous sensa-
tions, which seemed to come from the skin of the hand, but did not elicit any
motor responses; while in these same cases stimulation of the anterior central
Calcarine fissure-
Transition between striate
and peristriate areas
Cuneus -
Tangential fibers
^ - - -Stria of Gennari
-White center
Fig. 222. Section through the most rostral part of the cuneus. Pal-Weigert method.
gyrus gave rise to no sensations, but did call forth muscular contractions. On
the other hand, Head (1918), in a recent study of "Sensation and the Cerebral
Cortex," would include in the somesthetic area the anterior as well as the posterior
central convolution, and also the anterior part of the superior parietal lobule
and the angular gyrus. This study shows, perhaps better than any other work,
how intricate and difficult the problem of cortical localization really is and how
far we are from an ultimate solution.
The visual receptive center is located in the cortex forming the walls of the
calcarine fissure and in the adjacent portions of the cuneus and the lingual
gyrus (Figs. 217, 221). Rostral to the point where the calcarine is joined by the
parieto-occipital fissure the visual cortex is located only along the ventral side
of the former. Sometimes the center may extend around the occipital pole on
to the lateral surface of the brain (Fig. 216, Area 17). The structural peculiar-
ities of the visual cortex are very evident. It is not more than one-half as thick
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 293
as the motor cortex, and the outer line of Baillarger is greatly increased in thick-
ness and known as the line of Gennari (Fig. 218, C). Because of the prominence
of this line the region is known as the area striata. It is surrounded by cortex
of quite different structure; and nowhere can the differences in adjacent cortical
areas be better illustrated than at its border, where the prominent line of Gennari
is seen to terminate abruptly (Fig. 222). The fibers of the optic radiation from
the pulvinar and lateral geniculate body terminate in the visual projection center.
These fibers carry impulses from the temporal side of the corresponding retina
and the nasal side of the opposite one. The visual cortex of one hemisphere,
therefore, receives impressions from the objects on the opposite side of the line
of vision (Figs. 162, 163).
The auditory receptive center is located in the anterior transverse temporal
gyrus, which lies buried in the floor of the lateral sulcus. The area comes to
the surface near the middle of the dorsal border of the superior temporal gyrus
(Fig. 220). It receives the auditory radiation from the medial geniculate body.
The olfactory receptive center is located in the uncus and adjacent portions
of the hippocampal gyrus (principal olfactory area of Cajal). Within it ter-
minate the fibers of the lateral olfactory stria. They form a rather thick layer
of tangential fibers on its surface, which increases the thickness of the plexiform
layer.
Association Centers. It will be seen that the sensory and motor projection
centers occupy only a small part of the entire area of the cortex. The remaining
parts are connected with these centers by association fibers and are known as
association centers. Each area of sensory projection is surrounded by a zone
closely linked up with it by such fibers, and therefore probably under the dom-
inating influence of the particular sensory impulses reaching that projection
center. Their positions are indicated by lighter shading in Figs. 220 and 221.
Campbell (1905) has applied to them the designations "audito-psychic" and
"visuo-psychic fields" (Figs. 223, 224). The same author has designated
the portion of the frontal cortex immediately rostral to the motor projection
center the "intermediate precentral area," and is of the opinion it is especially
concerned with the "execution of complex movements of an associated kind,
of skilled movements, and of movements in which consciousness or volition takes
an active part." There still remains more than half of the cortical area, in-
dicated in white in Figs. 220 and 221, which is probably less intimately related
to any particular projection center. The fact that the increased size of the
human cerebral hemisphere over that of the higher apes is due to the much
294
THE NERVOUS SYSTEM
greater development of the association centers in man, suggests that these are of
especial significance for the higher intellectual functions.
VisuQ-tCHSOry
Fig. 223.
Fig. 224.
Figs. 223 and 224. Areas of the human cerebral cortex each of which possesses a distinctive
structure. (Campbell.)
In the present state of our knowledge of cortical activity and its relation to
consciousness it is the part of wisdom to be very conservative in locating any
mental faculty or fraction of our conscious experience in any particular part of
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 295
the cerebral cortex. We know upon which areas the auditory, visual, and olfac-
tory impulses impinge, and less accurately that in which the thalamic radiation,
mediating general bodily sensibility, terminates. Destruction of these areas
causes impairment or loss of the corresponding sensations with reference to the
opposite side of the body or the opposite half of the field of vision. Total loss
of cutaneous sensibility even within circumscribed areas never results from cor-
tical lesions; and it seems probable that the thalamic centers are in themselves
sufficient for a certain low grade, non-discriminative consciousness or awareness
of cutaneous stimulation. This is particularly true of painful sensations, which
seem to be for the most part of thalamic origin (Head, 1918). Furthermore,
the various parts of the cerebral cortex are so intimately linked together by as-
sociation fibers that when afferent impulses reach a given projection center they
must not only activate this center, but be propagated to other parts of the cortex
Motor speech center
Auditory speech center Visual s P eech center
Fie. 225. The cortical areas especially concerned with language.
as well. In view of these facts it is best to express the known facts of cortical
localization in terms of the relation of particular areas to the known projection
fiber systems.
Aphasia. Some idea of the significance of the so-called association centers
may be obtained from a study of the group of speech defects included under the
term "aphasia." In right-handed individuals these result from lesions in the
left hemisphere. Destruction of the triangular and opercular portions of the
inferior frontal gyrus usually causes loss of ability to carry out the coordinated
movements required in speaking, but does not impair the ability 'to move the
tongue or lips (Fig. 225). This defect is known as motor aphasia. Broca's
center, as this particular part of the cortex is sometimes called, is located in
Campbell's intermediate precentral area; and motor aphasia serves as a good
illustration of the importance of the entire intermediate precentral area for the
296 THE NERVOUS SYSTEM
execution of skilled volitional movements of an associated kind. In the same
way, after a lesion in the posterior part of the left superior temporal gyrus,
the patient may hear the spoken word, but no longer comprehend its meaning.
This is sensory aphasia or word deafness. Word blindness, the inability to under-
stand the printed or written language, although there is no impairment of vision,
may result from lesions in the angular gyrus. These three areas are often spoken
of as speech centers and are closely united together by association fibers. In
fact, it is not altogether clear to what extent such defects as those mentioned
above are dependent upon the destruction of these association tracts which lie
subjacent to the speech centers.
THE MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE
The medullary center of the cerebral hemisphere underlies the cortex and
separates it from the lateral ventricle and corpus striatum. It varies greatly
in thickness, from that of the thin lamina separating the insula and the claus-
trum (Fig. 191) to that of the massive centrum semiovale (Fig. 174). The
myelinated nerve-fibers of which it is composed are of three kinds, namely, as-
sociation fibers, projection fibers, and commissural fibers.
Commissural Fibers. As was stated in Chapter XV, there are three com-
missures joining together the cerebral hemispheres. Of these, the corpus callo-
sum is by far the largest and its radiation contributes largely to the bulk of the
centrum semiovale (Fig. 174). The fibers which compose it arise in the various
parts of the neopallium of each hemisphere; they are assembled into a broad
compact plate as they cross the median plane, and then spread out again to
terminate in the neopallium of the opposite side. As they spread through the
centrum semiovale they form the radiation of the corpus callosum. Some cor-
tical areas are better supplied with these fibers than others, few, if any, being
associated with the visual cortex about the calcarine fissure (Van Valkenburg,
1913). The majority of the callosal fibers do not connect together symmetric
portions of the cortex; but, after crossing the median plane, the fibers from a
given point in one hemisphere spread out to many parts of the opposite side.
The anterior and hippocampal commissures connect portions of the rhinencephalon
in one hemisphere, with similar parts on the opposite side. The anterior com-
missure connects together by its rostral part the two olfactory bulbs and by its
caudal part the two pyriform areas (Figs. 187, 194, 195). The hippocampal
commissure is composed of fibers which join together the two hippocampi by
way of the fimbriae and the psalterium.
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 297
Projection Fibers. Many of the fibers of the medullary white center connect
the cerebral cortex with the thalamus and lower lying portions of the nervous
system. These are known as projection fibers, and may be divided into two
groups according as they convey impulses to or from the cerebral cortex. The
corticipetal or afferent projection fibers include the following: (1) the optic radia-
tion, which arises in the pulvinar of the thalamus and the lateral geniculate
body and ends in the visual cortex about the calcarine fissure (Fig. 221); (2) the
auditory radiation, which arises in the medial geniculate body and terminates in
the auditory cortex of the anterior transverse temporal gyms; (3) the thalamic
radiation which unites the lateral nucleus of the thalamus with various parts of
the cerebral cortex, and which forms the ventral, frontal, and parietal stalks of
the thalamus (Fig. 195). The fibers of the parietal stalk include the sensory
fibers to the somesthetic cortex in the posterior central gyrus. The lateral ol-
factory stria, which conveys impulses from the olfactory bulb to the pyriform
area, is not a projection system in the strict sense of the word, since it begins
and ends within the telencephalon.
Efferent projection fibers convey impulses from the cerebral cortex to the
thalamus, brain stem, and spinal cord. They represent the axons of pyramidal
cells. The most important groups are those of the corticospinal and corticobulbar
tracts, which together form the great motor or pyramidal system. These fibers
begin in the motor cortex of the anterior central gyrus as axons of the giant cells
of Betz. Entering the white medullary center of the hemisphere, they are as-
sembled in the corona radiata (Fig. 194) and enter the internal capsule (Fig.
88). Their course beyond this point has been traced in the preceding chapters.
They convey impulses to the primary motor neurons of the opposite side of the
brain stem and spinal cord. Another important group of corticifugal fibers is
contained in the corticopontine tracts. Of these there are two main strands.
The frontopontine tract consists of fibers which begin as axons of cells in the cortex
of the frontal lobe, traverse the centrum semiovale, corona radiata, frontal part
of the internal capsule and medial one-fifth of the basis pedunculi, and finally
terminate in the nuclei pontis. The temporopontine tract has a similar origin
from the cortical cells of the temporal lobe and possibly of the occipital lobe also,
passes through the sublenticular part of the internal capsule and lateral one-
fifth of the basis pedunculi, and finally terminates in the nuclei pontis (Figs.
88, 106). The ascending thalamic radiation is paralleled by descending
corticothalamic fibers, which should be included among the efferent projection
systems, although their physiologic significance is not fully understood. Similar
298 THE NERVOUS SYSTEM
efferent fibers are contained in the optic radiation. They arise in the cortex
about the calcarine fissure and terminate in the pulvinar, lateral geniculate
body, and superior colliculus of the corpora quadrigemina (Fig. 162). A corti-
corubral tract descends from the frontal lobe through the posterior limb of the
internal capsule to end in the red nucleus of the mesencephalon. There do not
appear to be any strictly corticostriate fibers, but, according to Cajal (1911),
collaterals from the corticospinal fibers are given off to the corpus striatum.
The efferent projection tracts which we have considered all have their origin in
the neopallium.
There are several projection tracts from the rhinencephalon, and of these the
most important is the fornix. The fibers of this fascicle take origin in the hip-
Cingulu
Inferior longitudinal
fasciculus
Fig. 226. Some of the important association bundles projected upon the medial aspect of the
cerebral hemisphere. (Sobotta-McMurrich.)
pocampus, follow an arched course already described, and, entering the dien-
cephalon, terminate in part in the mammillary body and in part in the teg-
mentum of the brain stem (Fig. 205).
The frontal olfactory projection tract arises from the gray matter of the ol-
factory peduncle and the lateral olfactory gyrus. It enters the brain stem and
terminates in the pons and the medulla oblongata (Fig. 211).
Association Fibers. The various parts of the cortex within each hemisphere
are bound together by associatiorrfibers of varying length. The short associa-
tion fibers are of two kinds: (1) those which run in the deeper part of the cortex
and are designated as intracortical, and (2) those just beneath the cortex, which
are known as the subcortical fibers. The greater number of these subcortical
association fibers unite adjacent gyri, curving in U-shaped loops beneath the
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE
299
intervening sulci, and are accordingly often designated as arcuate fibers (Fig.
226). Others unite somewhat more widely separated gyri. The long association
fibers form bundles of considerable size, deeply situated in the medullary center
of the hemisphere, and unite widely separated cortical areas. There are five
of these which may be readily displayed by dissection of the human cerebral
hemisphere, namely, the uncinate, inferior occipitofrontal, inferior longitudinal,
and superior longitudinal fasciculi, and the cingulum. Another, known as the
fasciculus occipitofrontalis superior, is less easily displayed.
The cingulum is an arched bundle which partly encircles the corpus callosum
not far from the median plane (Figs. 174, 226). It begins ventral to the rostrum
of the corpus callosum, curves around the genu and over the dorsal surface of
Optk radiation External capsule and lentiform nucleus
Corona radiata / Frontal lobe
~--Fas. occipitofrontalis
inferior
~'Fas. uncinatus
"-Temporal lobe
Fig. 227. Lateral view of a dissection of a human cerebral hemisphere. The dorsal part
of the hemisphere has been cut away. On the lateral side the insula, opercula, and adjacent parts
have been removed.
that commissure to the splenium, and then bends ventrally to terminate near the
temporal pole. It is closely related to the gyrus cinguli and the hippocampal
gyrus and is composed for the most part of short fibers, which connect the various
parts of these convolutions.
The uncinate fasciculus connects the orbital gyri of the frontal lobe with the
rostral part of the temporal lobe. It is sharply bent on itself as it passes over
the stem of the lateral fissure of the cerebrtffii (Figs. 227, 228). The inferior
longitudinal fasciculus is a large bundle which runs through the entire length of
the temporal and occipital lobes (Fig. 226). It connects the occipital pole,
the cuneus, and other parts of the occipital lobe with the temporal cortex, ex-
tending as far forward as the temporal pole. According to Curran (1909) the
300
THE NERVOUS SYSTEM
uncinate and inferior longitudinal fascicles are formed by the shorter and more
superficial fibers of a larger and longer tract, the inferior occipitof rental fasciculus,
Superior longitudinal fasciculus
Uncinate fasciculus ''
Inferior occipitofrontal fasciculus
Fig. 228. Some of the long association bundles projected upon the lateral aspect of the cerebral
hemisphere.
which unites the cortex of the frontal and occipital lobes (Figs. 227, 228). Along
with the uncinate fasciculus it may easily be exposed by dissection, as it courses
along the ventrolateral border of the lentiform nucleus.
Cingulum
Fas. occipitofrontalis sup.
Corpus callosum
Fas. longitudinalis sup.
Caudate nucleus
Internal capsule
Lentiform nucleus
Insula
Fas. occipitofrontalis inf.
Fas. uncinatus
Amygdaloid nucleus
Fig. 229. Frontal section of the cerebral hemisphere through the anterior commissure showing the
location of the long association bundles.
The superior longitudinal fasciculus (fasciculus arcuatus) is a bundle of as-
sociation fibers which serves to connect many parts of the cortex on the lateral
THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE 301
surface of the hemisphere (Fig. 228). It sweeps over the insula, occupying the
base of the frontal and parietal opercula, and then bends downward into the
temporal lobe (Fig. 174). It is composed for the most part of bundles of rather
short fibers which radiate from it to the frontal, parietal, occipital, and temporal
cortex.
The superior occipitofrontal fasciculus runs in an arched course close to the
dorsal border of the caudate nucleus and just beneath the corpus callosum. It
is separated from the superior longitudinal fasciculus by the corona radiata
(Fig. 229).
The weight of the brain varies with the sex, age, and size of the individual.
The average weight of the brain in young adult men of medium stature is
1360 grams. It is less in women and in persons of small size or advanced age.
It is doubtful if there is any close correlation between the brain weight and
intelligence or between the latter and the size and arrangement of the cerebral
convolutions (Donaldson, 1898).
CHAPTER XIX
THE GREAT AFFERENT SYSTEMS
EXTEROCEPTIVE PATHWAYS TO THE CEREBRAL CORTEX
As has been intimated elsewhere, it is chiefly those nervous impulses, which
are aroused by stimuli acting upon the body from without, that rise above the
subconscious level and produce clear-cut sensations. The importance of these
sensations in our conscious experience is no doubt correlated with the fact that
it is through the reactions, called forth by such external stimuli, that the organism
is enabled to respond appropriately to the various situations in its constantly
changing environment. To meet these complex and variable situations cor-
rectly requires the nicest correlation of sensory impulses from the various sources
as well as their integration with vestiges of past experience, and it is in connec-
tion with these higher correlations and adjustments that consciousness appears.
The responses initiated by interoceptive and proprioceptive afferent impulses
are more stereotyped and invariable in character ; and these reactions are for the
most part carried out without the individual being aware either of the stimulus
or the response.
It is known that the cerebral cortex is the organ within which occur at least
the majority of those complex and highly variable correlations and integrations
which have consciousness as their counterpart. A single object may appeal
to many sense organs, and our perception of that object involves a synthesis of
a corresponding number of sensations and their comparison with past experience.
For example, when I meet a friend and grasp his hand in greeting, my perception
of him includes not only the image of his face but also the sound of his voice
and the warm contact of his hand. Thus thermal, tactile, auditory, and visual
sensations may be fused in the perception of a single object, and this involves an
integration of the corresponding afferent impulses within the cerebral cortex.
Accordingly, it becomes of special interest to trace the course of these afferent
impulses from the various exteroceptive sense organs to their cortical receptive
centers.
As we shall see, the outer world has for the most part a crossed representation
in the cerebral cortex. Cutaneous stimuli, received from objects touching the
302
THE GREAT AFFERENT SYSTEMS
right side of the body, and optic stimuli produced by light waves coming from
the right half of the field of vision, are propagated to the cortex of the left hemi-
sphere. The crossed representation in the case of hearing is less complete, partly
because every sound wave reaches both ears, but also because the crossing of
the central auditory pathway seems to be incomplete.
The grouping of the afferent fibers in the peripheral nerves differs from that
in the spinal cord. In each of the spinal nerves several varieties of sensory fibers
are freely mingled. In the cutaneous branches are found conductors of thermal,
tactile, and painful sensibility; while the deeper nerves contain fibers for pain
and sensations of pressure-touch as well as for muscle, joint, and tendon sensi-
bility. Because of the intermingling of the various kinds of fibers a lesion of a
spinal nerve results in a loss of all modalities of sensation in the area supplied
exclusively by that nerve.
But in the spinal cord a regrouping of the afferent impulse occurs, such that
all of a given modality travel in a path by themselves. All those of touch and
pressure, whether originally conveyed by the superficial or deep nerves, find
their way into a common path in the cord. In the same way all painful impulses,
whether arising in the skin or deeper parts, follow a special course through the
cord. Another intramedullary path conveys impulses from the muscles, joints,
and tendons. These various lines of conduction within the cord are so distinct
from each other that a localized spinal lesion may interrupt one without affecting
the others. A striking illustration of this is the loss of sensibility to pain and
temperature over part of the body surface without any impairment of tactile
sensibility as a result of a disease of the spinal cord, known as syringomyelia.
While we shall here confine our attention to the afferent channels leading
directly toward the cerebral cortex, it should not be forgotten that these are in
communication with the reflex apparatus of all levels of the spinal cord and brain
stem.
The Spinal Path for Sensations of Touch and Pressure. Tactile impulses
which reach the central nervous system by way of the spinal nerves are relayed
to the cerebral cortex by a series of at least three units.
Neuron I. The first neuron of this conduction system has its cell body,
which typically is unipolar, located in the spinal ganglion; and its axon divides
in the manner of a T or Y into a central and a peripheral branch. The per-
ipheral branch runs through the corresponding spinal nerve to the skin, or in
the case of those fibers subserving the tactile functions of deep sensibility, to the
underlying tissues. The central branch from the stem process of the spinal
THE NERVOUS SYSTEM
ganglion cell enters the spinal cord by way of the dorsal roots. The touch fibers
are probably myelinated and enter the cuneate fasciculus through the medial
division of the dorsal root; and, like all other dorsal root fibers, they divide into
ascending and descending branches. The ascending branches run for varying
distances in the posterior funiculus, giving off collaterals before they terminate
Internal capsule
~ Thalamus
Spinothalamic tract ~
Ascending branches of
dorsal root fibers
Ventral Spinothalamic tract
Mesencephalon
Medulla oblongata
Medial lemniscus -|-
Spinal cord
Dorsal root and spinal ganglion
Fig. 230. Diagram of the tactile path.
in the gray matter of the spinal cord, some few at least even reaching the nucleus
gracilis and cuneatus in the medulla oblongata. At varying levels they enter
the gray substance of the columna posterior and form synapses with the neurons
of the second order (Fig. 230).
THE GREAT AFFERENT SYSTEMS
Neuron II, with its cell body located in the posterior gray column, sends its
axon across the median plane into the ventral spinothalamic tract in the opposite
anterior funiculus. In this the fiber ascends through the spinal cord and brain
stem to the thalamus. This tract gives off fibers, either collateral or terminal,
to the reticular formation of the brain stem. Other neurons of the second order
in the tactile path are located in the gracile and cuneate nuclei of the medulla
oblongata, and their axons after crossing the median plane ascend in the median
lemniscus of the opposite side to end in the thalamus. All of these secondary
tactile fibers end within the ventral part of the lateral thalamic nucleus.
The course of the ventral spinothalamic tract through the medulla oblongata and pons
is not accurately known. It has generally been figured as joining the lateral spinothalamic
tract dorsolateral to the olive (Fig. 230. See also Herrick, Fig. 81). But, since lesions in
the lateral area of the medulla oblongata may cause a loss of pain and temperature sensation
over the opposite half of the body without affecting tactile sensibility, it is not improbable
that Dejerine (1914) is correct in supposing that it follows a median course, its fibers inter-
mingled with those of the tectospinal tract which run, however, in the opposite direction
(Fig. 234; Economo, 1911; Spilkr, 1915).
There is reason to believe that the ventral as well as the lateral spinothalamic tract
consists in part of short relays with synaptic interruptions in the gray matter of the spinal
cord and brain stem, and the two tracts are sometimes designated as the spino-reticulo-thala-
mic path.
In the spinal cord there appear to be two tracts which convey tactile im-
pulses toward the brain, an uncrossed one in the posterior funiculus and another
that crosses into the opposite anterior funiculus. Since these overlap each
other for many segments, this arrangement would account for the fact that con-
tact sensibility is usually unaffected by a purely unilateral lesion (Head and
Thompson, 1906; Rothmann, 1906; Petren, 1902). Among the fibers of contact
sensibility, which ascend in the posterior funiculus to the cuneate and gracile
nuclei of the same side, are those that subserve the function of tactile discrim-
ination, or, in other words, the ability to recognize the duality of two closely
juxtaposed points of contact, as when the two points of the compasses or dividers
are applied simultaneously to the skin. Furthermore, those elements of tactile
sensibility, which underlie the appreciation of the form of objects or stereognosis,
ascend uncrossed in the posterior funiculus to the gracile and cuneate nuclei.
Neuron III. The neurons located in the ventral portion of the lateral nucleus
of the thalamus, with which the tactile fibers of the second order enter into syn-
aptic relations, send their axons by way of the thalamic radiation through the
posterior limb of the internal capsule and the corona radiata to the somesthetic
area of the cerebral cortex in the posterior central gyrus (Fig. 220).
THE NERVOUS SYSTEM
THE SPINAL PATH FOR PAIN AND TEMPERATURE SENSATIONS
Pain and temperature sensations are mediated by closely associated though
not identical paths, and it is convenient to consider them at the same time.
Neuron I. The first neuron of this system has its cell of origin located in
the spinal ganglion. Its axon divides into a peripheral branch, directed through
Internal capsule
Thalamus
X Mesencephalon
Medulla oblongata
- Lateral spinothalamic tract
Spinal card
Dorsal root and spinal ganglion
Fig. 231. Diagram of the path for pain and temperature sensations.
the peripheral nerve to the skin, or in the case of the pain fibers also to the deeper
tissues, and a central branch, which enters the spinal cord through the dorsal
root and almost at once terminates in the gray matter of the posterior gray column
(Fig. 231). As was shown in Chapter VII, there is reason to believe that the
THE GREAT AFFERENT SYSTEMS
307
fibers of painful sensibility, and possibly those of temperature sensations as
well, are unmyelinated and enter the cord through the lateral division of the
dorsal root to end in the substantia gelatinosa Rolandi.
Neuron II. From these dorsal root fibers the impulses are transmitted
(perhaps through the intermediation of one or more intercalated neurons) to the
neurons of the second order. These have their cell bodies located in the pos-
terior gray column, and their axons cross the median plane and ascend in the
lateral spinothalamic tract to end in the ventral part of the lateral nucleus of
the thalamus. In addition to this long uninterrupted path, there probably
also exists a chain of short neurons with frequent interruptions in the gray
matter of the spinal cord, which serves as an accessory path to the same end
station. In the medulla oblongata the spinothalamic tract lies dorsolateral to
the inferior olivary nucleus. In the pons it joins the medial lemniscus and
runs in the lateral part of this fillet through the pons and mesencephalon to the
thalamus (Figs. 231, 234).
Neuron III. Fibers, arising from nerve-cells located in the lateral thalamic
nucleus, convey thermal and possibly also painful impulses to the somesthetic
area of the cerebral cortex in the posterior central gyrus by-way of the thalamic
radiation, and the posterior limb of the interal capsule. It is important to
note that it is not necessary for painful afferent impulses to reach the cerebral
cortex before they make themselves felt in consciousness, the thalamus being
in itself sufficient for the perception of pain (Head and Holmes, 1911 ; Head, 1918).
The Exteroceptive Paths Associated with the Trigeminal Nerve. The tri-
geminal nerve mediates tactile, thermal, and painful sensations from a large part
of the cutaneous and mucous surfaces of the head. While there is reason to be-
lieve that the tactile impulses mediated by this nerve follow a central course
distinct from that of thermal and painful sensibility, we cannot as yet assign
definite paths to either group, and shall consider the exteroceptive connections
of this nerve as a unit.
Neuron I. The axon of a unipolar cell in the semilunar ganglion divides
into a peripheral branch, distributed to the skin or mucous membrane of the
head, and a central branch, which runs through the sensory root (pars major)
of the trigeminal nerve into the pons. Here it divides into a short ascending
and a long descending branch. The former terminates in the main sensory
nucleus, and the latter in the spinal nucleus of that nerve (Fig. 232).
Neuron II. The fibers of the second order in the sensory paths of the tri-
geminal nerve arise from cells located in the main sensory and the spinal nucleus
38
THE NERVOUS SYSTEM
of that nerve; and after crossing the raphe they run in two tracts to the ventral
part of the lateral nucleus of the thalamus. The ventral secondary afferent
path is located in the ventral part of the reticular formation, close to the spino-
thalamic tract in the medulla oblongata and dorsal to the medial lemniscus in
the pons and mesencephalon (Figs. 132, 234). The dorsal tract lies not far
from the floor of the fourth ventricle and the central gray matter of the cerebral
'Medial lemniscus
Mesencephalon
o'o^ 1 *~o j- Medial lemniscus
Pons
Dorsal secondary tract N. V
Ventral secondary tract, N. V
^- Main sensory nucleus N. V
Pons
N. V
-- Spinal tract N. V
Spinal nucleus N. V
Medulla oblongata
Fig. 232. Diagram of the exteroceptive pathways associated with the trigeminal nerve.
aqueduct. It consists in considerable part of uncrossed fibers and of fibers hav-
ing a short course (Wallenberg, 1905; Economo, 1911; Dejerine, 1914).
Neuron III. The afferent impulses are relayed from the thalamus to the
cortex of the posterior central gyrus by fibers of the third order, which run through
the posterior limb of the internal capsule. Their cells of origin are located in
the lateral nucleus of the thalamus.
THE GREAT AFFERENT SYSTEMS
309
The Neural Mechanism for Hearing. The spiral organ of Corti within the
cochlea is connected with the auditory center in the cerebral cortex by a chain
of three or more units.
Neuron I. The bipolar cells of the spiral ganglion within the cochlea send
each a peripheral process to end in the spiral organ of Corti. Each sends a central
branch to ramify in the cochlear nuclei, where it forms sy nap tic connections
with the auditory neurons of the second order (Fig. 233).
Transverse temporal gyrus
Auditory radiation
Medial genicnlate body
Inferior cotticulus
v Lateral lemnisci
Collaterals to nucleus of
lateral lemniscus
/Strife medullares
,-Dorsal cochlear nucleus
-Ventral cochlear nucleus
Cochlear nerve
r Vestibular nerve
Rostral portion of the pons-/- (
!~\
J !
Caudal portion of the pons-\^
Superior olive ''
Trapezoid body '
Nucleus of the trapezoid body
Fig. 233. Diagram of the auditory pathway. (Based on the researches of Cajal and Kreidl.)
Neuron II. The cells located in the ventral and dorsal cochlear nuclei give
rise to fibers, which after crossing the median plane form the lateral lemniscus
of the opposite side. Those from the ventral cochlear nucleus cross the pons in
the trapezoid body, giving off collaterals to the superior olivary nuclei and the
nuclei of the corpus trapezoideum, and may be joined by fibers taking origin in
these nuclei. Lateral to the contralateral superior olivary nucleus they turn
abruptly rostrad in the lateral lemniscus. The fibers from the dorsal cochlear
nucleus run in the striae medullares of the fourth ventricle, and then, dipping
310 THE NERVOUS SYSTEM
into the reticular formation of the pons, cross the median raphe to join the trape-
zoid body and enter the lateral lemniscus. While this tract is for the most part
a crossed one, some fibers probably enter the lateral lemniscus from the cochlear
nuclei of the same side. This accounts for the fact that it is very rare to have
total deafness in either ear resulting from damage to the auditory pathway
within the brain. The fibers of this fillet give off collaterals to the nucleus of
the lateral lemniscus, from which some additional fibers may be contributed to
the tract, which finally terminates in the medial geniculate body and the inferior
colliculus of the corpora quadrigemina. The latter, however, serves only as a
reflex center, while the medial geniculate body is the way station on the
auditory path to the cerebral cortex.
Neuron III. Through synapses in the medial geniculate body the auditory
impulses are transferred to neurons of the third order, whose cell bodies are
located in this nucleus and whose fibers run through the auditory radiation
and the retrolenticular part of the internal capsule to the auditory receptive
center in the cerebral cortex. It will be remembered that this center is situated
in the anterior transverse temporal gyms, located upon the dorsal surface of
the temporal lobe within the lateral cerebral fissure, and in the small portion
of the superior temporal convolution with which that gyrus is directly continuous.
The Neural Mechanism for Sight. The nervous impulses responsible for
vision travel over a conduction system composed of at least four units. Since
this mechanism has already been considered as a whole on pages 225-228
it is only necessary for us to enumerate here the separate units of which it is
composed (Figs. 160, 162).
Neuron I. Visual cells of the retina including the rods and cones, which are
differentiated as receptors for photic stimuli.
Neuron II. Bipolar cells of the retina, forming synapses with the visual
cells, on the one hand, and the ganglion cells on the other.
Neuron III. Ganglion cells of the retina, whose axons enter the optic nerve,
undergo a partial decussation in the optic chiasma, and end in the lateral genic-
ulate body, pulvinar of the thalamus, and superior colliculus of the corpora
quadrigemina.
Neuron IV. From cells in the lateral geniculate body and the pulvinar of
the thalamus axons run by way of the optic radiation through the retrolenticular
part of the internal capsule to the visual receptive center in the cerebral hemi-
sphere. This is located in the cortex on both sides of the calcarine fissure and
occupies portions of the cuneus and the lingual gyrus.
THE GREAT AFFERENT SYSTEMS 311
PROPRIOCEPTIVE PATHWAYS
We have traced the course of the afferent impulses from the skin and from
the eye and ear to the cerebral cortex, and have learned that they play an es-
pecially important part in conscious experience. The stimulation of these ex-
teroceptive sense organs initiates both conscious and reflex adjustments of the
body to its environment. But the resulting movements serve to excite the
sensory nerve ending in the muscles, joints, and tendons; and any quick move-
ment or change in position of the head will also excite the nerve terminals in
the semicircular canals of the ear. From these sources afferent impulses pour
back into the nervous system along special paths to centers which to a great
extent are separate from those devoted to the exteroceptive functions and serve
to regulate the movements already initiated. The necessity for such regulation
is well illustrated by the ataxic gait of a tabetic in whom the afferent impulses
from the muscles, joints, and tendons are more or less completely lost. In a
sense the proprioceptive functions of the nervous system are secondary to the
exteroceptive, since the purpose of both is the proper adjustment of the organism
to its environment by means of reactions, called forth by external stimuli,
but regulated and controlled through afferent impulses arising within the
body.
Since in the regulation of movement the proprioceptive subdivision of the
nervous system has to deal with constant factors, inherent in the arrangement
of the muscles, the resultant responses are more stereotyped and invariable in
character and are, for the most part, subconsciously executed. These reactions
belong more to the province of the cerebellum than to that of the cerebrum.
Of the long ascending channels mediating afferent impulses from the muscles,
joints, and tendons, only one extends to the cerebral cortex by way of the thala-
mus; all the others end in the cerebellum. In fact, the cerebellum is the great
correlation center for afferent impulses of the propriceptive group, whether they
are conveyed by the vestibular nerve or the muscular branches of the spinal
nerves.
It will be understood that on the motor side these two subdivisions of the
nervous system are not as distinct as on the afferent side. On the contrary,
both tend to discharge into common efferent systems. This is particularly true
of the primary somatic motor neuron, which serves as "the final common path"
for both.
The Spinal Proprioceptive Path to the Cerebral Cortex. The conduction
system, along which those afferent impulses travel which underlie the rather
312
THE NERVOUS SYSTEM
Fig. 234. Diagrams showing the location of the most important tracts of the brain stem
based on figures by Dejerine. Solid red, aberrant bundles of the corticobulbar tract; red stipple,
corticospinal tract; solid blue, secondary afferent paths of the trigeminal nerve; horizontal blue
lines, the medial lemniscus (proprioceptive) ; blue stipple, ventral spinothalamic tract (or tactile
path); blue circles, spinal root of the trigeminal nerve; solid black, lateral spinothalamic tract
(pain and temperature); black triangles, ventral spinocerebellar tract; black circles, dorsal spino-
cerebellar tract; black stipple, rubrospinal tract. A, Through the mesencephalon at the level of
the inferior colliculus; B, through the rostral part of the pons; C, through the medulla at the level
of the olive.
THE GREAT AFFERENT SYSTEMS 313
vague sensations of position and posture and of active and passive movements,
consists of a chain of at least three units.
Neuron I. The cell bodies of the neurons of the first order belonging to this
system are located in the spinal ganglia. Their axons are myelinated and divide
into peripheral branches, running to specialized end organs within the muscles,
joints and tendons, and central branches directed through the medial division
of the dorsal root into the posterior funiculus of the spinal cord. Here they
divide; and their ascending branches run through the posterior funiculus to
terminate in the gracile and cuneate nuclei of the medulla oblongata, where they
enter into synaptic relations with neurons of the second order (Fig. 235).
Neuron II. From cells located in the gracile and cuneate nuclei the axons
run as internal arcuate fibers across the median raphe in the medulla oblongata
and ascend by way of the medial lemniscus to end in the ventral part of the lateral
nucleus of the thalamus, where they form synapses with neurons of the third order.
Neuron III. From cells in the lateral nucleus of the thalamus fibers pass by
way of the thalamic radiation through the posterior limb of the internal capsule
to the posterior central gyrus or somesthetic area of the cerebral cortex.
SPINAL PROPRIOCEPTIVE PATHS TO THE CEREBELLUM
Impulses from the muscles, joints, and tendons may reach the cerebellum by
three routes:
A. By Way of the Dorsal External Arcuate Fibers :
Neuron I of this chain is the same as in the path to the cerebral cortex just
described, the fibers from the dorsal root reaching the gracile and cuneate nuclei.
Neuron II. From cells located in these nuclei axons run as posterior external
arcuate fibers to the restiform body of the same side, and thence through the
white center of the cerebellum, to end in the cerebellar cortex (Fig. 235, red).
B. By Way of the Ventral Spinocerebellar Tract:
Neuron I. The first neuron in this chain is similar to the primary neuron in
the two preceding paths. The impulses, however, travel over collateral and
terminal branches of the dorsal root fibers to reach the posterior gray column
and intermediate gray matter of the spinal cord.
Neuron II. From cells located in the posterior gray column and intermediate
gray matter fibers run in the ventral spinocerebellar tracts of the same or
opposite side through the spinal cord, medulla oblongata and pons, bend around
the brachium conjunctivum, and then course back along the anterior medullary
velum to the cortex of the rostral part of the vermis (Fig. 235, blue).
THE NERVOUS SYSTEM
C. By Way of the Dorsal Spinocerebellar Tract:
Neuron I. The first neuron of this chain is similar to the primary neuron
in the. three preceding paths. The impulses, however, travel over those col-
lateral and terminal branches of the dorsal root fibers which ramify about the
cells of the nucleus dorsalis.
Internal capsule
Thalamus
Ventral spinocerebellar tract
Cerebellum
Restiform body
Medulla oblongata
Ascending branches of
dorsal rool fibers
--Dorsal external arcuate fiber
Ventral spinocerebellar tract
Dorsal spinocerebellar tract
Dorsal root and spinal ganglion
Fig. 235. The proprioceptive paths.
Neuron II. From cells in the nucleus dorsalis fibers run to the dorsal spino-
cerebellar tract of the same side and through the restiform body to the cortex
of both the rostral and the caudal portions of the vermis (Fig. 235, red).
Cerebellar Connections of the Vestibular Nerve. The vestibular nerve
THE GREAT AFFERENT SYSTEMS 315
conducts impulses from specialized sense organs in the semicircular canals, sac-
cule and utricle, which are stimulated by movements and changes in posture
of the head.
Neuron I. From the bipolar cells of the vestibular ganglion (of Scarpa),
located within the internal auditory meatus, peripheral processes run to the
maculae of the utricle and saccule and to the cristae of the semicircular canals.
The central processes are directed through the vestibular nerve toward the
floor of the fourth ventricle and divide into ascending and descending branches.
While the descending and many of the ascending branches terminate in the
vestibular nuclei, many other ascending branches pass without interruption to
end in the cerebellar cortex and particularly in that of the vermis (Fig. 136).
Neuron II. Some of the cells situated in the vestibular nuclei send their
axons, along with the ascending branches mentioned above in the vestibulo-
cerebellar tract, to the cortex of the vermis, and to a less extent to the cortex
of the cerebellar hemispheres also.
CHAPTER XX
EFFERENT PATHS AND REFLEX ARCS
THE motor apparatus is a complex mechanism into which the pyramidal
system enters as a single factor. The primary motor neurons of the brain stem
and spinal cord are also under the influence of other motor centers than those
found in the cerebral cortex. They receive impulses from the corpora quadri-
gemina through the tectospinal tract, from the lateral vestibular nucleus by way
of the vestibulospinal tract, from the large motor cells of the reticular formation
through the reticulospinal path, from the cerebellum, and probably also from
the corpus striatum by way of the red nucleus and the rubrospinal fasciculus.
Perhaps, also, impulses descend from the thalamus or subthalamus by way of a
thalamospinal tract.
We must not think of the individual parts of this complex mechanism as
functioning separately, since each of these motor centers contributes its share
to the control of the primary motor neuron, upon which as the "final common
path" all these efferent pathways converge. Only by keeping this fact con-
stantly in mind can the motor functions be properly understood. The same
idea has been well stated by Walshe (1919): 'Tn stimulation experiments on the
motor cortex we see a complex motor mechanism at work under the influence
of an abnormally induced, crude form of hyperactivity of the predominant partner
in this mechanism. Conversely, after destructive lesions, we observe it at work
liberated from the control of this predominant partner and deprived of its actual
cooperation."
On the other hand, the grave motor disturbances resulting from lesions in
the basal ganglia and especially the corpus striatum with little or no involvement
of the corticospinal tracts (paralysis agitans, Auer and McCough, 1916; bilateral
athetosis, Cecile Vogt, 1911; and progressive lenticular degeneration, Wilson,
1912-14) have recently called attention to the importance of the corpus striatum
and the extrapyramidal motor path (see p. 324). In these diseases voluntary
movements are impeded by tremor, rigidity, and athetosis; and in all probability
these disturbances arise because the pyramidal system is deprived of the co-
operation of one of the subordinate "partners" in the motor combine.
316
EFFERENT PATHS AND REFLEX ARCS
Even after cerebral control has been entirely eliminated in the dog by de-
cerebration, many reflex functions remain, which represent the unguided activity
of the lower elements in the motor mechanism ; and we now know that a similar
independent reflex activity may occur in the spinal cord of man after total trans-
verse lesions (Riddoch, 1917).
THE GREAT MOTOR PATH
The great motor path from the cerebral cortex to the skeletal musculature,
through which the bodily activities are placed directly under voluntary control,
is in man and mammals the dominant factor in the motor mechanism. We
have seen that afferent channels from the various exteroceptors reach the cere-
Fig. 236. Cortical localization upon the lateral aspect of the human cerebral hemisphere. (Starr.)
bral cortex; and that through the correlation of the olfactory, auditory, visual,
tactile, thermal, and painful afferent impulses which pour into it, there is built
up within the cortex a representation of the outer world and its constantly chang-
ing conditions. The responses appropriate to meet the entire situation in which
the individual finds himself from moment to moment are in large part at least
initiated in the cerebral cortex and are executed through the motor mechanism.
In these responses the great motor path is the dominant factor, although other
parts of the mechanism are secondarily called into action, especially the pro-
prioceptive reflex arcs, including the coordinating and tonic mechanism of the
cerebellum.
This great motor path consists of two-unit chains. The so-called upper
motor neurons conduct impulses from the motor cortex to the motor nuclei of the
THE NERVOUS SYSTEM
cerebral nerves or to the anterior gray columns of the spinal cord; whence the
lower motor neurons, also known as primary motor neurons, relay the impulses
to the muscles. It is possible that another and much shorter element is inter-
calated between the two chief units of this conduction system.
The motor cortex occupies the rostral lip of the central sulcus and the ad-
jacent portion of the anterior central gyrus, extending over the dorsal border of
Motor cortex
...Posterior limb of internal capsule
. Genu of corpus cattosum
Basis pedunculi of mesencephalon
----^Longitudinal fascicles of pons
Pyramid of medulla ablongata
Lateral corticospinal tract
Ventral corticospinal tract
Fig. 237. The corticospinal path.
the hemisphere into the paracentral lobule. Within this area the skeletal mus-
culature is represented hi inverted order, that moving the toes near the dorsal
border of the hemisphere. The area from which the corticobulbar tract arises
is only a small part of the whole, and is situated near the lateral cerebral fissure
(the region marked Eyelids, Cheeks, Jaws, Lips in Fig. 236). From all the rest
of the motor cortex arise the fibers of the corticospinal tract.
EFFERENT PATHS AND REFLEX ARCS
319
The motor path for the spinal nerves includes the corticospinal tract and the
spinal primary motor neurons.
Neuron I, or upper motor neuron. The giant pyramidal cells of the motor
cortex give rise to the fibers of the corticospinal tract which is also known as
Fissura longitudinalis cerebri
Radiatio corporis callosi,.
Septum pellucidum x
Plexus chorio-
ideus ventricul
lateralis
Corona radiata,.
Columna,
fornicis
'Plexus chorio-
ideus ventriculi ~"~V
tertii
Capsula interna
Thalamus
Ventriculus -
tertius
Fossa inter-
peduncularis
(Tarini)
Cornu inferius
ventriculi
lateralis
Peduncuh
cerebri
Brachium pontj
Fasciculi longit
nales(pyramida
pontis
Facies inferio
, Gyrus frontalis superior
f Truncus corporis callosi
( Cornu anterius ventriculi
lateralis
Caput nuclei caudati
Nn.facialisund
acusticus
Flocculus
glossopharyngctis
N. vagus
Nucleus olivaris inferior
Fibrae pontis super
Pyramis medullae oblongatae / ^ Decussatio pyramidum
Fig. 238. Section through the brain in the axis of the brain stem, showing the entire extent of
the corticospinal tract. (Toldt.)
the cerebrospinal fasciculus or pyramidal tract. These fibers traverse the rostral
half of the posterior limb of the internal capsule, the intermediate three-fifths
of the basis pedunculi, the basilar portion of the pons, and the pyramid of the
medulla oblongata, and after undergoing a partial decussation are continued into
the spinal cord (Figs. 237, 238). At the pyramidal decussation in the caudal
320
THE NERVOUS SYSTEM
part of the medulla oblongata the greater part of the tract crosses to the opposite
side of the spinal cord and is continued as the lateral corticospinal tract in the
lateral funiculus. The smaller part is continued directly into the ventral fu-
niculus of the same side, as the ventral corticospinal tract. The fibers of the
ventral tract cross the median plane a few at a time and terminate, as do those of
the lateral tract, directly or indirectly in synaptic relations with the primary
motor neurons within the anterior gray column (Fig. 239). The ventral tract
is not evident as a well-marked bundle below the level of the midthoracic region.
Mesencephalon
N.IV
Pans
Corticobulbar tract
Medulla oblongata
Ventral corticospinal tract
Lateral corticospinal tract
Spinal cord
Ventral root
Fig. 239. The corticobulbar and corticospinal tracts.
It has long been known that in the higher mammals the lateral pyramidal tract, although
consisting predominatingly of crossed fibers, contains a few homolateral fibers also (Simpson,
1902), and according to the observations of Dejerine (1914) and other investigators this
holds true for man also. Dejerine speaks of these uncrossed fibers in the lateral corticospinal
tract as a third bundle arising out of the motor decussation, and calls it the "homolateral"
corticospinal fasciculus. A good account of this tract and of the superficially placed bundle
of uncrossed pyramidal fibers that is to be found in the ventral part of the lateral funiculus
in the cervical portion of the spinal cord is given by Barnes (1901).
Neuron II. To the lower or primary motor neurons belong the large multi-
polar cells of the anterior gray column of the spinal cord. These give rise to the
motor fibers that leave the spinal cord through the ventral roots and are dis-
tributed through the spinal nerves to the skeletal musculature.
The motor path for the cranial nerves is less well known. It includes the
corticobulbar tract and those fibers of the cranial nerves which innervate striated
musculature.
Neuron I, or upper motor neuron. The corticobulbar fibers arise from the
EFFERENT PATHS AND REFLEX ARCS
321
giant pyramidal cells of the part of the motor cortex near the lateral fissure.
These fibers run through the genu of the internal capsule and the basis pedunculi
to end, directly or indirectly, in synaptic relation to the primary motor neurons
of the somatic motor and special visceral motor nuclei of the brain stem. Be-
fore terminating, the majority cross the median plane, but some end in the motor
nuclei of the same side (Fig. 239).
Neuron II, lower or primary motor neuron. From the large multipolar
cells of the somatic motor and special visceral motor nuclei arise fibers, which
run through the cranial nerves to end in striated musculature.
Tr. corticosp.
Tr. corticobulb.
F. A. Sth. (Ill)
F. A.Pd. / Tr. cb. lot.
(Ill, VI,XI)\Tr. cb.med.
F. A.P. (V, X, XI, XII)
F.A.B.P. (VII)
Tr. corticosp.
Tr. corticobulb.
X I, C II-IV
C 1 1 -IV
Tr. corticosp. med.
C II-IV
XI, C II-IV
Puhinar
Med. lemniscus
Nuc. N. Ill
Corpora quad.
Nuc. N. IV
Nuc. N. V
Fourth vent.
Nuc. N. VI
Nuc. N. VII
Nuc. ambiguus Nn. IX and X
Nuc. N. XII
Nuc. gracilis
Nuc. cuneatus
Nuc. XI
XI, XII, C II-IV
Tr. corticosp. lot.
Fig. 240. The course of the fibers of the corticobulbar tract. Redrawn from Dejerine.
Corticobulbar tract, solid black; corticospinal tract, vertical lines; the medial lemniscus, horizontal
lines. F. A. B. P., Bulbopontine aberrant fibers; F. A. P., aberrant fibers of the pons; F. A. Pd.,
aberrant fibers of the peduncle; F. A. Sth., subthalamic aberrant fibers; Tr. cb. lat., tractus cortico-
bulbaris lateralis; Tr. cb med., tractus corticobulbaris medialis. The Roman numerals indicate
the nuclei of the cranial and cervical nerves which are supplied by the various bundles.
The Corticobulbar Tract. According to Dejerine (1914), who, because of the careful
study which he and his associates have made of this efferent system, is most entitled to speak
authoritatively on the subject, the corticobulbar fibers occupy chiefly the medial part of the
basis pedunculi and its deeper layer. The fibers separate into two major groups. One
part follows the course of the corticospinal tract and descends in the basilar portion of the
pons and the pyramids of the medulla oblongata. Another part, which he designates as
the system of aberrant pyramidal fibers, detaches itself from the preceding in small bundles
at successive levels of the brain stem. These enter the reticular formation and descend
within the region occupied by the medial lemniscus, giving off fibers to the motor nuclei of
the cranial nerves (Fig. 240). The fibers undergo an incomplete decussation in the raphe
322 THE NERVOUS SYSTEM
and go chiefly to the nuclei of the opposite side. The decussating fibers are grouped in very
small bundles, those for a given nucleus crossing at the level of that nucleus. There is great
variation in the course of the bundles of aberrant pyramidal fibers in different brains.
The chief aberrant bundles which can be traced dorsalward into the reticular formation
(indicated in solid red in Fig. 234) are as follows:
1. The aberrant fibers of the peduncle (Fig. 240, F. A. Pd.) form two bundles, which
have been called by some authors the median and lateral corticobulbar tracts. These
descend in the territory of the medial lemniscus (Figs. 234, 240) and give off fibers to the
nuclei of the third, sixth, and eleventh cranial nerves. With these two bundles run some
fibers destined for the upper cervical segments of the spinal cord. This group of aberrant
fibers therefore controls the movements of the eyes and the associated movements of the head.
2. The aberrant fibers of the pons (Fig. 240, F. A. P.) which join the preceding in the
medial lemniscus run to the motor nuclei of the trigeminal and hypoglossal nerves and to the
nucleus ambiguus.
3. The bulbopontine aberrant fibers (Fig. 240, F. A. B. P.) leave the main trunk of the
pyramidal system near the level of the sulcus between the pons and medulla. They reinforce
the preceding groups, supply the motor nucleus of the facial nerve, and send fibers to the
nucleus ambiguus and to that of the hypoglossal nerve.
These facts are of the greatest importance for the clinical neurologist. Lesions re-
stricted to the basilar portion of the pons are likely to destroy at the same time the cortico-
spinal fibers and those of the corticobulbar tract which end in the facial nucleus. A lesion
confined to the reticular formation and involving the medial lemniscus may, according to its
level, sever the corticobulbar fibers for the motor nuclei of the eye-muscle nerves or those
for the motor nuclei of the trigeminal, accessory, and hypoglossal nerves without involve-
ment of the corticospinal tracts. Conjugate deviation of the head and eyes, not often seen
as a result of damage to the basilar portion of the pons, may result from tegmental lesions
involving the aberrant fibers of the peduncle.
The physiologic and clinical significance of the course of the corticospinal and
corticobulbar tracts is obvious. It is because of the decussation of these fibers
that the muscular contractions produced by cortical stimulation occur chiefly
on the opposite side of the body, and that the paralyses resulting from lesions
in the pyramidal system above the decussation are contralateral. If the lower
motor neuron is injured, the associated muscle atrophies and a flaccid paralysis
results. Injury to the upper motor neuron, on the other hand, leads to a loss
of function without atrophy, but rather with an increased tonicity of the affected
muscle, i. e., to a spastic paralysis. By means of such differential characteristics
as these it is possible to tell which of the two links in the motor chain has been
broken.
In order to understand the combination of symptoms, which result from
damage to the motor path at different levels, it is necessary to have in mind the
topography of its constituent parts. Some of these relations are indicated in
Fig. 241. Since the motor cortex is spread out over a rather extensive area,
it is usually not entirely destroyed by injury or disease. A restricted cortical
EFFERENT PATHS AND REFLEX ARCS
323
lesion may cause a monoplegia, i. e., paralysis of a single part, such as the arm or
leg (Fig. 241, A). But in the internal capsule the motor fibers are grouped
within a small area and are frequently all destroyed together. This causes
paralysis of the opposite half of the body or hemiplegia (Fig. 241, B). Damage
to the pyramidal system in the cerebral peduncle, pons, or upper part of the
medulla oblongata may also cause hemiplegia; but in such cases those cortico-
To the arm
To the leg
Fig. 241. Diagram to illustrate the effects of lesions in various parts of the motor path.
bulbar fibers, which leave the main strand of pyramidal fibers above the level
of the lesion, may escape injury and the corresponding cranial nerves need not
be involved (Fig. 241, C). Furthermore, in lesions of the brain stem the motor
nucleus or emergent fibers of one of the cranial nerves may be destroyed along
with the pyramidal fibers, in which case there would result a paralysis of the
muscles supplied by that nerve as well as a paralysis of the opposite half of the
body below that level a crossed paralysis (Fig. 241, C). While damage to the
324 TTTF. NERVOUS SYSTEM
spinal cord may affect only one lateral half and cause a homolateral paralysis
below the lesion (Fig. 241, D), it is common for both lateral halves to be involved
and for the resulting paralysis to be bilateral (Fig. 241, ).
The Extrapyramidal Motor Paths. In recent years it has become increasingly evi-
dent that the pyramidal system is not the only channel through which volitional impulses
are able to reach the primary motor neurons of the brain stem and spinal cord. Rothmann
(1907) found that, after section of the lateral corticospinal and the rubrospinal tracts in
monkeys at the level of the third cervical nerve, voluntary movements were lost for a time,
but soon reappeared; and he concluded that there must be an extrapyramidal volitional
path in the ventral funiculus. Three years later Schafer (1910) showed that in monkeys
the paralysis, which results from section of the pyramids of the medulla oblongata, is not
complete and persistent; and he agreed with Rothmann that there must be some other path
for volitional impulses. He believes that this alternative path is formed by descending
fibers in the ventral funiculus and in the ventral part of the lateral funiculus, since section
of these fibers produces as complete and persistent paralysis in monkeys as does section of
the pyramids themselves.
Sherrington and Graham Brown (1913) excised the arm area of the cerebral cortex
in the chimpanzee, and found that function in the corresponding limb was completely re-
stored in a few weeks. They were able to show that this was not attributable to the vicarious
activity of the corresponding postcentral or the opposite precentral cortex. Horsley's (1909)
patient, who recovered some degree of control over the arm after the removal of its cortical
center in the precentral gyrus, shows that the observations of Sherrington and Brown are at
least in part applicable to man.
We know that the cerebral cortex is connected through efferent projection tracts with
the thalamus and red nucleus and through collaterals from the corticospinal fibers with the
corpus striatum (Cajal). But we do not know which, if any, of these systems of projection
fibers constitutes a part of the extrapyramidal path for volitional impulses.
A great deal of attention has recently been given by clinical neurologists to the dis-
turbance of voluntary movement by tremor, rigidity, and athetosis, which results from lesions
of the corpus striatum. This body seems to contain an important motor center, and ac-
cording to Wilson (1912 and 1914) it exerts a steadying influence upon voluntary move-
ments. The globus pallidus seems to be connected with the spinal primary motor neurons
by way of the striorubral and rubrospinal tracts. It is also possible, especially in view
of the important motor functions attributed to the ventrolateral descendnig tracts of the
spinal cord by Rothmann and Schafer, that efferent impulses reach the spinal cord from the
globus pallidus by way of the substantia nigra over the strionigral, the somewhat hypothetic
nigroreticular, and the reticulospinal tracts. It is known that the axons arising in the sub-
stantia nigra run into the reticular formation of the mesencephalon, beyond which they
cannot be traced (Cajal, 1911). According to Collier and Buzzard (1901) the rubrospinal,
vestibulospinal, tectospinal, and reticulospinal tracts probably represent the original paths
for impulses from higher to lower parts of the nervous system; and the path from the
cerebrum to the spinal cord, at first indirect, has been short-circuited in the mammal
through the evolution of the pyramidal system.
When it is remembered that the pyramidal system is a late development, present only
in mammals, it does not seem unreasonable to think that some other and older path for
volitional impulses may also exist. The globus pallidus, the representative of the primitive
corpus striatum of the lower vertebrates, has been called the paleostriatum (Elliot Smith,
1919). From this basal nucleus there arises in all vertebrates an important efferent bundle,
EFFERENT PATHS AND REFLEX ARCS
325
"the basal forebrain bundle" of Edinger (1887), which is represented in mammals by the
striofugal fibers of the ansa lenticularis. It is clear that this fascicle, which persists through-
out the vertebrate series, must subserve important functions; and it is probable that it
forms a part of the extrapyramidal motor path.
THE CORTICO-PONTO-CEREBELLAR PATH
The cortico-ponto-cerebellar path is an important descending conduction
system which places the cerebellum under the influence of the cerebral cortex.
Since a part of the corticopontine fibers are collaterals given off to the nuclei
of the pons by the corticospinal fibers, and since in many mammals practically
Red nucleus
\Purkinje cell
i Cerebellum
Frontopontine tract
Corticospinal tract
Nuclei pontis
Muscle
Spinal cord
- Dentate nucleus
~ Brachium conjunctivum
" * Brachium pontis
Rubrospinal tract
Corticospinal tract
Fig. 242. The cortico-ponto-cerebellar and cerebello-rubro-spinal paths. (Modified from Cajal.)
all of the corticopontine fibers are represented by such collaterals (Cajal, 1909),
one can scarcely avoid the conclusion that through this system the coordinating
and tonic mechanism of the cerebellum is brought into play for the regulation
of movements initiated from the cerebral cortex. In this sense the idea of
Cajal (1911) that there exists an indirect motor path to the spinal cord through
the cerebellum is probably correct (Fig. 242).
Neuron I. From pyramidal cells in the frontal lobe of the cerebral cortex
fibers pass through the anterior limb of the internal capsule and the medial one-
326 THE NERVOUS SYSTEM
fifth of the basis pedunculi; and similar fibers from the temporal lobe descend
through the sublenticular part of the internal capsule and the lateral one-fifth
of the basis pedunculi. These fibers, together with the corticospinal tract,
form the longitudinal fasciculi of the pons; and, along with collaterals from that
tract, they end within the nuclei pontis in synaptic relations with the neurons
of the second order (Figs. 106, 242).
Neuron II. Arising from cells in the nuclei pontis, the transverse fibers of
the pons cross the median plane and run by way of the brachium pontis and
white substance of the cerebellum to the cerebellar cortex of the opposite side.
THE CEREBELLO-RUBRO-SPINAL PATH
The cerebello-rubro-spinal path is the conduction system through which the
cerebellum contributes its important share to the control of the primary motor
neurons of the spinal cord in the interest of muscular coordination, equilibration,
and the maintenance of muscle tone. Other efferent connections of the cerebel-
lum have been discussed on page 211.
Neuron I. From the Purkinje cells of the cerebellar cortex fibers run to
terminate in the central nuclei of the cerebellum, especially the dentate nucleus
(Fig. 242).
Neuron II. Arising chiefly, if not entirely, from the cells of the dentate
nucleus, fibers run through the brachium conjunctivum, undergo decussation
in the tegmentum of the midbrain ventral to the inferior colliculi, and end in the
red nucleus and thalamus (Figs. 242, 243).
Neuron III. From cells in the red nucleus arise the fibers of the rubrospinal
tract, which cross the median plane in the ventral tegmental decussation, and
descend through the reticular formation of the brain stem and the lateral funic-
ulus of the spinal cord. Here this tract occupies a position just ventral to the
lateral corticospinal tract, and its fibers end in the anterior gray column in
relation to the primary motor neurons.
We have learned that the cerebellum is the chief center of the proprioceptive
system and is concerned with the maintenance of the proper tonicity of the
muscles, the coordination of their contractions, and especially with those re-
actions necessary to maintain or to re-establish that evenly balanced spacial
orientation known as equilibrium. The cerebello-rubro-spinal path is the con-
duction system primarily concerned in these reactions.
What is perhaps the first direct experimental evidence of the function of this
system has been given by Weed (1914). The extensor rigidity, so characteristic
EFFERENT PATHS AND REFLEX ARCS
327
of decerebrated dogs, which Sherrington (1906) clearly showed to be a proprio-
ceptive reflex that under normal conditions serves to keep the limbs from bend-
ing under the weight of the body, is apparently dependent upon the integrity of
the cerebello-rubro-spinal path. Weed showed that removal of the cerebellum,
section of the superior cerebellar peduncles, or transection of the mesencephalon
below the level of the red nucleus obliterated or greatly decreased this rigidity.
Rubrospinal tract ^
Rubroreticular tract
From frontal lobe and corpus striatum
" Thalamus
I Red nucleus
Brachium conjunctivum
' Dentate nucleus
Pons
Rubrospinal tract
J Medulla oblongata
u
Reticulospinal tract
Spinal cord
Fig. 243. Diagram showing the connections of the red nucleus: A, Ventral tegmental
decussation; B, decussation of the brachium conjunctivum; C and D, descending fibers from bra-
chium conjunctivum, before and after its decussation respectively.
On the other hand, stimulation of the area occupied by the red nucleus on the
cut surface of the mesencephalon in decerebrated dogs increased the rigidity.
IMPORTANT REFLEX ARCS
We have considered the afferent paths leading to the cerebral cortex and to
the cerebellum as well as the efferent channels which conduct impulses from these
centers to the skeletal musculature. But there are many more direct paths
by which impulses may travel from receptor to effector, and these are known as
reflex arcs. It will be worth while to review briefly a few of the more important
of these rather direct receptor to effector circuits.
328 THE NERVOUS SYSTEM
REFLEX ARCS OF THE SPINAL CORD
Neuron I. Primary sensory neurons, with cell bodies in the spinal ganglia,
convey impulses from the sensory endings to the spinal cord, then along the
ascending and descending branches resulting from the bifurcation of the dorsal
root fibers within the cord, and along the collaterals of these branches to the
primary motor neurons, either directly or through an intercalated central unit
(Figs. 66-68).
Neuron II. The central neurons have their cell bodies in the posterior gray
column and may belong to Golgi's Type II, having short axons restricted to the
gray matter; or their axons may be long, running through the fasciculi proprii
to the ventral horn cells at other levels of the cord. Some of these central axons
cross the median plane in the anterior commissure.
Neuron III. Primary motor neurons, with cell bodies in the anterior gray
column, send their axons through the ventral roots and spinal nerves to the
skeletal musculature. Or in the case of visceral reflexes, the motor neuron has
its cell body located in the intermediolateral cell column, and its axon runs as a
preganglionic fiber to a sympathetic ganglion, whence the impulses are relayed
by a fourth or postganglionic neuron to involuntary muscle or glandular tissue.
The reflex paths of the cranial nerves are similarly constituted, except that
rarely if ever do the sensory fibers form synapses directly with the motor cells.
The central neuron, which has its cell located in the sensory nucleus of a given
nerve, sends its axon through the reticular formation to the motor nucleus of
the same or of some other nerve (Figs. 92, 111). Two of the reflex circuits con-
nected with the vestibular nerve require special attention.
VESTIBTJLAR REFLEX ARC THROUGH THE MEDIAL LONGITUDINAL BUNDLE
Neuron I. The bipolar cells of the vestibular ganglion in the external audi-
tory meatus send peripheral processes to the cristae of the semicircular canals
and maculae of the saccule and utricle. Their central processes run through
the vestibular nerve to the vestibular nuclei (Figs. 135, 244).
Neuron II. Cells in the lateral and superior vestibular nuclei send their axons
to the medial longitudinal fasciculus of the same or the opposite side, where they
divide into ascending and descending branches, which run in this bundle. From
these branches twigs are given off to the nuclei of the oculomotor, trochlear, and
abducens nerves and to the motor cells of the cervical portion of the spinal cord
(Fig. 244).
Neuron III. Primary motor neurons of the oculomotor, trochlear, abducens,
EFFERENT PATHS AND REFLEX ARCS
329
accessory, and cervical spinal nerves send their axons to the muscles that move
the head and eyes.
This arc is concerned with the reflex regulation of the combined movements
of the head and eyes in response to the vestibular stimulation which results from
every movement and change of posture of the head. Strong stimulation of the
semicircular canals, vestibular nerve, or Deiters' nucleus causes an oscillatory
side to side movement of the eyes, known as nystagmus, a reflex response of an
abnormal character mediated through this arc (Wilson and Pike, 1915).
M. rectus medialis
Oculomotor nerve
Vestibular nerve '
Lateral vestibular nucleus -
Vestibules pinal tract '
Median longitudinal --'
fasciculus
M. sternocleidomas- _
toideus
M. rectus lateralis
Nuc. of oculomotor nerve
Abducens nerve
Nuc. of abducens nerve
Median longitudinal
fasciculus
Spinal root of accessory
nerve
t N. ceroicalis II
Fig. 244. Vestibular reflex arcs. (Modified after Edinger.)
A vestibules pinal reflex -arc is established between the vestibular sense organs
and the skeletal musculature and consists of the following parts : the vestibular
nerve; the vestibulospinal tract, which has its origin in the lateral vestibular
nucleus, and descends in the ventral funiculus of the same side of the spinal
cord; and the primary motor neurons of the spinal cord (Fig. 244).
The afferent impulses reaching the medulla oblongata by way of the vagus
give rise to a great variety of reflexes. While these are for the most part purely
visceral, a few are executed by the somatic musculature and should receive
attention at this point.
33
THE NERVOUS SYSTEM
The Respiratory Reflex Mechanism. The maintenance of the normal res-
piratory rhythm is dependent upon a respiratory center in the caudal part of
the medulla oblongata, which is sensitive to changes in the carbon dioxid con-
tent of the blood. But this rhythm is also influenced by afferent impulses coming
from the lungs by way of the vagus nerve and the tractus solitarius. It is
probable that these impulses are relayed through the nucleus of the tractus soli-
tarius and descending fibers that arise in that nucleus (tractus solitariospinalis)
to the primary motor neurons belonging to the phrenic and intercostal nerves
(Fig. 245). There must also be a descending tract from the respiratory center
to these neurons. Cajal (1909) believes that this center is, in fact, identical
with the lower part of the nucleus of the tractus solitarius (the commissural
Dorsal motor X nucleus
Nucleus offascic. solitarius
Fasciculus solitarius
Vagus ganglion
Vagus nerve
Tr. solitario-spinalis
Sympathetic ganglion
Blood-vessel
Respiratory center
Intercostal nerve
Intercostal muscle
Phrenic nerve
Diaphragm
Fig. 245. Reflex mechanism of respiration. (Herrick, Cajal.)
nucleus), and that this responds both to changes in the chemical composition of
the blood and to the afferent impulses coming by way of the vagus nerve. If
this be true, the fibers from the nucleus of the tractus solitarius would be the
only descending tract needed to carry the respiratory impulses to the spinal
cord. Although on its afferent side the respiratory reflex is visceral, it is ex-
ecuted by somatic muscles which are under voluntary control; and hence breath-
ing may be temporarily suspended or the rhythm altered at will.
The reflex mechanism for vomiting and coughing is illustrated in Fig. 246.
As the result of an irritation of the gastric mucous membrane a wave of excitation
travels along the afferent fibers of the vagus nerve and the tractus solitarius.
After passing through synapses in the nucleus of that tract, the impulses probably
EFFERENT PATHS AND REFLEX ARCS
331
travel along the descending fibers, which arise in that nucleus, to the primary
motor neurons of the spinal cord that give rise to the fibers innervating the dia-
phragm and abdominal muscles. At the same time the musculature of the
stomach is excited to contraction by that part of the wave of excitation which
reaches the dorsal motor nucleus of the vagus. These impulses reach the mus-
culature of the stomach over the visceral efferent fibers of the vagus and an
intercalated postganglionic neuron.
A similar neural circuit is probably responsible for reflex coughing. From
the irritated respiratory mucous membrane, as, for example, of the larynx, the
Vagus ganglion
Intercostal muscle
Diaphragm
Stomach
Dorsal motor vagus
nucleus
Nucleus of fasciculus
solilarius
Fasciculus solitarius
Tr. solitariospinalis
Phrenic nerve
Intercostal nerve
Nerve to abdominal
muscles
Sympathetic ganglion
Postganglionic
Fig. 246. Reflex mechanism of coughing and vomiting. (Herrick, Cajal.)
disturbance is propagated along the afferent fibers of the vagus, through the
nucleus of the tractus solitarius and the descending fibers arising in it to the
spinal primary motor neurons, which innervate the diaphragm and the inter-
costal and abdominal muscles.
The corpora quadrigemina are important reflex centers. The path for re-
flexes in response to sound begins in the spiral organ of Corti and follows the coch-
lear nerve and its central connections, including the lateral lemniscus, to the
inferior colliculus of the opposite side, and to a less extent of the same side also
332
THE NERVOUS SYSTEM
(see p. 309). Thence the path follows the tectospinal and tectobulbar tracts
to the primary motor neurons of the cerebrospinal nerves (see p. 167). The
visual reflex arc begins in the retina, follows the optic nerve and optic tract with
partial decussation in the chiasma, to the superior colliculus of the corpora
quadrigemina (p. 226) ; thence it is continued by way of the tectospinal and tecto-
bulbar paths to the primary motor neurons of the cerebrospinal nerves (Fig. 162).
Pupillary Reactions. The iris is innervated by two sets of sympathetic
nerve-fibers derived from the ciliary and the superior cervical sympathetic ganglia
respectively. Impulses reaching the iris through the latter ganglion induce
dilatation of the pupil; those through the ciliary ganglion cause constriction.
The latter reaction always accompanies accommodation. When vision is fo-
N.II
Ciliary ganglion
N.
Sup. colliculus
Sensory nuc. N. V
Pons-
Upper thoracic segments of <
spinal cord
N. V
\ Carotid plexus
Sup. cervical sympatltetic ganglion
"- Cervical sympatlielic trunk
Fig. 247. Pupillary reflex arcs.
cused on a near object, contraction of the ciliary muscle results in accommoda-
tion; and at the same time contraction of the two internal rectus muscles brings
about a convergence of the visual axes. These two movements are always
associated with a third, the contraction of the sphincter pupillae. In addition
to this constriction of the pupil, which accompanies accommodation, two other
pupillary reactions require attention (Fig. 247).
The Pupillary Reflex (Light Reflex) When light impinges on the retinae
there results a contraction of the sphincter pupillae and a corresponding constric-
tion of the pupil. The reflex circuit, which is traversed by the impulses bringing
about this reaction, begins in the retina and includes the following elements:
the fibers of the optic nerve and tract, with a partial decussation in the optic
EFFERENT PATHS AND REFLEX ARCS 333
chiasma; synapses in the superior colliculus of the corpora quadrigemina ; fibers
of the tectobulbar tract ending in the nucleus of Edinger-Westphal (visceral
efferent portion of the oculomotor nucleus); the visceral efferent fibers of the
oculomotor nerve, ending in the ciliary ganglion; and the postganglionic fibers
extending from the ciliary ganglion to iris.
The pupillary-skin reflex is a dilatation of the pupil following scratching of
the skin of the cheek or chin. This is but one example of the fact that dilatation
of the pupil can be induced by the stimulation of many sensory nerves and con-
stantly occurs in severe pain. The path includes the following parts : the fibers
of these sensory nerves and their central connections in the brain stem and spinal
cord; preganglionic visceral efferent fibers, which arise from the cells of the inter-
mediolateral column of the spinal cord and run through the upper white rami
and the sympathetic trunk to the superior cervical sympathetic ganglion; and
postganglionic fibers, which arise in that ganglion and run through the plexus on
the internal carotid artery to end in the iris (Fig. 247).
We have in the case of the pupillary reactions an illustration of the double
and antagonistic innervation, which, as we shall see in the next chapter, is a
rather characteristic feature of the autonomic nervous system.
CHAPTER XXI
THE SYMPATHETIC NERVOUS SYSTEM
THE sympathetic nervous system is an aggregation of ganglia, nerves, and
plexuses, through which the viscera, glands, heart, and blood-vessels, as well as
Ciliary ganglion Maxillary nerve
Sphenopalaline ganglion v
Superior cervical ganglion of sympathetic \ \
Cervical plexus
Brachial plexus
Greater splanchnic nerve
Lesser splanchnic nerve
Lumbar plexus
Sacral plexus
Pharyngeal plexus
Middle cervical ganglion of sympathetic
Inferior cervical gang, of sympathetic
Recurrent nerve
Bronchial plexus
Cardiac plexus
Esophageal plexus
^Coronary plexui
Left vagus nerve
Gastric plexus
Celiac plexus
Superior mesenteric plexus
Aortic plexus
Inferior mesenteric plexus
I
Hypogastric plexus
Pelvic plexus
Bladder
Vesical plexus
. ^
Fig. 248. The sympathetic nervous system. (Schwalbe, Herrick.)
smooth muscle in other situations, receive their innervation. As illustrated in
Fig. 248 it is widely distributed over the body, especially in the head and neck
334
THE SYMPATHETIC NERVOUS SYSTEM
335
and in the thoracic and abdominal cavities. It must not be too sharply de-
limitated from the cerebrospinal nervous system, since it contains great numbers
of fibers which run to and from the brain and spinal cord. For example, the
vagus nerve contains many fibers which are distributed through the thoracic
and abominal sympathetic plexuses for the innervation of the viscera. In the
same way the spinal nerves are connected by communicating branches or rami
communicates with the sympathetic trunks.
The sympathetic trunks are two nerve cords which extend vertically through
the neck, thorax, and abdomen, one on each side of the vertebral column (Fig.
248). Each trunk is composed of a series of ganglia arranged in linear order
and bound together by short nerve strands. Every spinal nerve is connected
with the sympathetic trunk of its own side by one or more gray rami commu-
nicantes through which it receives fibers from the sympathetic trunk. Fibers
reach this trunk from the thoracic and upper lumbar nerves by way of the white
rami communicantes (Fig. 257). The sympathetic trunk also gives off branches
which enter into the formation of the nerve plexuses which are associated with
the larger arteries. The largest of these plexuses is the celiac, which is associ-
-ated with the upper portion of the abdominal aorta and its branches. In this
plexus and located in close relation to the abdominal aorta are the celiac,
mesenteric, and aorticorenal ganglia, all of which are in man grouped hi a pair
of large irregular masses designated as the celiac ganglia and placed one on
either side of the celiac artery (Fig. 257). The sympathetic ganglia may be
grouped into three series as follows: (1) the ganglia of the sympathetic trunk,
arranged in linear order along each side of the vertebral column and joined
together by short nerve strands to form the two sympathetic trunks; (2) col-
lateral ganglia, arranged about the aorta and including the celiac and mesenteric
ganglia; and (3) terminal ganglia, located close to or within the structures
which they innervate. As examples of the latter group there may be men-
tioned the ciliary and cardiac ganglia and the small groups of nerve-cells in
the myenteric and submucous plexuses (Fig. 257).
FUNDAMENTAL FACTS CONCERNING VISCERAL INNERVATION
General visceral afferent fibers are found in the ninth and tenth cranial
nerves and in many of the spinal nerves, especially in those associated with the
white rami (thoracic and upper lumbar nerves) and in the second, third, and
fourth sacral nerves. These afferent fibers take origin from cells in the cerebro-
spinal ganglia (Fig. 249). From these ganglia the fibers run through the corres-
336 THE NERVOUS SYSTEM
ponding cerebrospinal nerves to the sympathetic nervous system, through which
they pass without interruption in any of its ganglia to end in the viscera. These
fibers are of all sizes, including large and small myelinated fibers and many which
are unmyelinated (Chase and Ranson, 1914; Ranson and Billingsley, 1918).
The afferent impulses mediated by these fibers serve to initiate visceral re-
flexes, and for the most part remain at a subconscious level. Such general vis-
ceral sensations as we do experience are vague and poorly localized. Tactile
sensibility is entirely lacking in the viscera and thermal sensibility almost so,
although sensations of heat and cold may be experienced when very warm or
cold substances enter the stomach or colon (Carlson and Braafladt, 1915).
Pain cannot be produced by pinching or cutting the thoracic or abdominal
viscera. Acute visceral pain may, however, be caused by disease, as in the pas-
sage of a stone along the ureter.
From the cerebrospinal ganglia the visceral afferent impulses are carried to the brain
and spinal cord by the sensory nerve roots. The relations within the cerebrospinal ganglia
are not entirely clear; but it seems probable that the visceral afferent impulses are conducted
through the ganglion by way of the two branches of the typical unipolar sensory neuron
(Fig. 249). Many authors believe that there are also sensory fibers which arise from cells
in the sympathetic ganglia and terminate in the spinal ganglia in the form of pericellular
plexuses (Fig. 40, C). Through these plexuses visceral sensory impulses are supposed to be
transmitted to somatic sensory neurons and to be relayed by them to the spinal cord. Since
it has not been clearly demonstrated that any sensory fibers arise from cells in the sym-
pathetic ganglia, this interpretation of the pericellular plexuses of the spinal ganglia must be
regarded as purely hypothetic.
Langley (1903) has presented strong evidence that few if any sensory fibers arise in the
sympathetic ganglia. Physiologic experiments show that the visceral afferent fibers run in
the white rami, yet all or practically all of the fibers of a white ramus degenerate if the cor-
responding spinal nerve is severed distal to the spinal ganglion. Huber (1913) states that
"it has not been determined that the fine medullated fibers or the unmedullated fibers which
appear to enter the spinal ganglia from without and end in pericellular plexuses are, in
fact, the neuraxes of sympathetic neurones." The hypothesis that these pericellular plexuses
represent the termination of visceral afferent fibers is, therefore, not well supported. This
subject is treated in more detail in a series of papers on the sympathetic nervous system by
Ranson and Billingsley (1918).
Visceral Efferent Neurons. The general visceral efferent fibers of the
cerebrospinal nerves take origin from cells located within the cerebrospinal axis.
They do not run without interruption to the structures which they innervate;
instead, they always terminate in sympathetic ganglia, whence the impulses,
which they carry, are relayed to their destination by neurons of a second order
(Fig. 249). This important information we owe to Langley (1900 and 1903),
who showed that the injection of proper doses of nicotin into rabbits prevents
THE SYMPATHETIC NERVOUS SYSTEM
337
the passage of impulses through the sympathetic ganglia, although an undi-
minished reaction may be obtained by stimulation of the more peripheral sym-
pathetic nerves By a long series of experiments Langley has shown that there
are always two and probably never more than two neurons concerned in the
conduction of an impulse from the central nervous system to smooth muscle
or glandular tissue. The neurons of the first order in this series are designated as
preganglionic, those of the second order as postganglionic, with reference to the
relation which they bear to the ganglion containing their synapse.
Preganglionic neurons have their cell bodies located in the visceral efferent
column of the cerebrospinal axis. The cells of this series are smaller than those
Spinal ganglion
Dorsal ramus
,' Ventral ramus
Ramus communicans
--- Sympathetic ganglion
<y\ Visceral efferent fiber
Somatic efferent fiber
<^1 Postganglionic fiber
root
' ______ ,Viscus
Fig. 249. Diagrammatic section through a spinal nerve and the spinal cord in the thoracic region
to illustrate the chief functional types of peripheral nerve-fibers.
of the somatic motor column and contain less massive Nissl granules. From
these cells arise the fine myelinated visceral efferent fibers which run through
the cerebrospinal nerves to the sympathetic nervous system and terminate in
the sympathetic ganglia (Fig. 249) .
Postganglionic neurons have their cell bodies located in the sympathetic
ganglia. In fact, these cells with their dendritic ramifications and the terminal
branches of the preganglionic fibers synaptically related to them are the es-
sential elements in the sympathetic ganglia. Their axons for the most part
remain unmyelinated and run as Remak fibers through the sympathetic nerves
338
THE NERVOUS SYSTEM
s
O J2
C/5 '-5
J3
S J3
THE SYMPATHETIC NERVOUS SYSTEM 339
and plexuses, to end in relation with involuntary muscle or glandular tissue.
A very few postganglionic fibers acquire delicate myelin sheaths.
Three streams of preganglionic fibers leave the cerebrospinal axis (Fig. 250).
The cranial stream includes the general visceral efferent fibers of the oculomotor,
facial, glossopharyngeal, vagus, and accessory nerves. These fibers end in the
terminal ganglia, already mentioned, which are located close to or within the
organ which they innervate. In the cervical nerves there are no visceral ef-
ferent fibers, the cranial stream being separated from the next by a rather wide
gap. The thoracicolumbar stream includes the fibers which arise from the cells
of the intermediolateral column of the spinal cord and make their exit through
the thoracic and first four lumbar nerves (Langley, 1892; Miiller, 1909). After
leaving the spinal nerves by way of the white rami they enter the sympathetic
nervous system and terminate in the ganglia of the sympathetic trunk or in the
celiac and associated collateral ganglia (Fig. 250). The sacral stream includes
the visceral efferent fibers of the second, third, and fourth sacral nerves. These
arise from cells in the lateral column of gray matter in the sacral portion of the
spinal cord and run through the visceral branch of the third sacral and a similar
branch from either the second or fourth sacral nerves. These fibers end in the
ganglia of the pelvic sympathetic plexuses.
The Autonomic Nervous System. For many reasons it is convenient to have
a name which will designate the sum total of all general visceral efferent neurons,
both preganglionic and postganglionic, whether associated with the cerebral
or spinal nerves. For this purpose the term "autonomic nervous system" is
in general use. It designates that functional division of the nervous system 1
which supplies the glands, heart, and smooth musculature with their efferent in- I
nervation (Fig. 250). It is important to bear in mind that this is a functional
and not an anatomic division of the nervous system, that it includes only efferent
elements, and that the preganglionic neurons lie in part within the cerebrospinal
nervous system. The terminal portions of the preganglionic fibers and the
postganglionic neurons are located in the sympathetic system. According to
the origin of the preganglionic fibers, we may recognize the following three
subdivisions of the autonomic system: (1) the cranial autonomic system, whose
preganglionic fibers make their exit by way of the third, seventh, ninth, tenth,
and eleventh cranial nerves; (2) the thoracicolumbar autonomic system, whose pre-
ganglionic fibers make their exit by way of the thoracic and upper lumbar spinal
nerves; and (3) the sacral autonomic system, whose preganglionic fibers run in
the visceral rami of the second, third, and fourth sacral nerves (Fig. 250).
040 THE NERVOUS SYSTEM
The fibers of the thoracicolumbar stream run by way of the white rami to
the sympathetic trunk, while the fibers of the cranial and sacral streams make
no connection with that trunk, but run directly to the sympathetic plexuses.
And while the thoracicolumbar preganglionic fibers terminate hi the ganglia of
the trunk, those of cranial and sacral origin end in the terminal ganglia. In
these two respects the cranial and sacral streams agree with each other and differ
from the thoracicolumbar outflow. Also in their response to certain drugs,
like atropin and adrenalin, the two former agree with each other and differ from
the latter. It is, therefore, desirable to group the cranial and sacral systems
together as the craniosacral autonomic system. This has been called by many
physiologists the parasympathetic system. It stands in contrast to the thoracico-
lumbar autonomic system to which many physiologists have unfortunately applied
the name "sympathetic system." The importance of recognizing these two
principal subdivisions is further emphasized by the fact that most of the struc-
tures innervated by the autonomic system receive a double nerve supply and are
supplied with fibers from both subdivisions. The thoracicolumbar fibers are
accompanied in most peripheral plexuses by craniosacral fibers of opposite func-
tion so that the analysis of these plexuses is greatly facilitated by subdividing
the autonomic system in this way.
Visceral Reflexes. In the gastro-intestinal tract and perhaps within other
viscera there may be a mechanism for purely local reactions as indicated in
the following paragraph. With this exception the evidence strongly indicates
that all visceral reflex arcs pass through the cerebrospinal axis. In such an
arc there are at least three neurons, namely, (1) visceral afferent, (2) pregang-
lionic visceral efferent, and (3) postganglionic visceral efferent neurons (Fig. 249) .
The purely local reactions which occur in the gut wall after section of all of
the nerves leading to the intestine are known as myenteric reflexes and must de-
pend upon a mechanism different from that of other visceral reflexes (Langley
and Magnus, 1905; Cannon, 1912). Practically nothing is known of this mech-
anism beyond the fact that it must be located in the enteric plexuses. Some
authors have assumed that within these plexuses there is a diffuse nerve net
similar to that found in the ccelenterates (Parker, 1919). While the evidence
is far from satisfactory, it may be that such a net does exist in this situation and
that it is responsible for these local reactions.
THE SYMPATHETIC NERVOUS SYSTEM
STRUCTURE OF THE SYMPATHETIC GANGLIA
341
The nerve-cells of the sympathetic ganglia are almost all multipolar, but there
are also a few that are unipolar or bipolar. Each cell is surrounded by a nucleated
membranous capsule. Some of the dendrites ramify beneath this capsule and
are designated as intracapsular. Others pierce the capsule, run long distances
through the ganglia, and are known as extracapsular dendrites.
Fig. 251. Neurons from the human superior cervical sympathetic ganglion (pyridin-silver
method): A, Three nerve cells and the intercellular plexus: a, unicellular glomerulus; b, neuron
with extracapsular dendrites. B, Tricellular glomerulus. C, Neuron surrounded by subcapsular
dendrites.
Intracapsular dendrites are numerous in the sympathetic ganglia of man,
but rare in those of mammals (Marinesco, 1906; Cajal, 1911; Michailow, 1911;
Ranson and Billingsley, 1918). Beneath the capsule these dendrites may form
an open network more or less uniformly distributed around the cell (Fig. 251, C),
or they may be grouped on one side of the cell, causing a localized bulging in
the capsule (Fig. 251, A, a). Such a localized mass of subcapsular dendrites
with interlacing branches is known as a glomerulus. Following CajaPs classifi-
cation we may distinguish four types of glomeruli according to the number of
342
THE NERVOUS SYSTEM
neurons whose dendrites enter into their formation, namely, unicellular (Fig.
25 1 , A , a) , bicellular , triceUular (Fig. 25 1 , B) , and multicellular glomeruli. Short
intracapsular dendrites with swollen ends are sometimes present in the sym-
pathetic ganglia of mammals (Fig. 252, A}.
Fig. 252. Sympathetic ganglion cells showing various types of dendrites. Redrawn from
Michailow. Methylene-blue stain. A, From superior mesenteric ganglion, horse; B, from celiac
ganglion, horse; C, from stellate ganglion, horse; D, from superior cervical ganglion, dog; E, celiac
ganglion, horse; F, superior cervical ganglion, dog.
Extracapsular dentrites pierce the capsule, run for longer or shorter dis-
tances among the cells, and help to form an intercellular plexus of dendritic and
axonic ramifications (Fig. 251, -4). These dendrites may end in a variety of
ways. Some of these types of endings may be enumerated as follows: (1)
brush-like endings (Fig. 252, A); (2) plate-like or bulbous terminals applied
THE SYMPATHETIC NERVOUS SYSTEM
343
against the outer surface of the capsule of another cell (Fig. 252, B, C) ; (3) inter-
lacing branches, which form a plexus upon the outer surface of the capsule of
an adjacent cell (Fig. 252, D}.
Dogiel (1896) thought that the cells possessing the longest dendrites were sensory, but
Cajal (1911) could find no evidence for this, and was unable to trace any of them from the
ganglia and associated nerves to the viscera. Carpenter and Conel (1914), using the size
and arrangement of the Nissl granules as a criterion,- were able to find only one cell type in
the sympathetic ganglia, and concluded that these ganglia do not contain sensory nerve-cells.
Fig. 253. Neurons and intercellular plexus from the superior cervical sympathetic ganglion of a
dog (pyridin-silver method).
The axons of sympathetic ganglion cells are usually unmyelinated, but a few
of them acquire thin myelin sheaths. They are the postganglionic fibers which
relay the visceral efferent impulses to the innervated tissue. According to
Cajal (1911), who states that his anatomic studies are in accord with the physio-
logic experiments of Langley, the axons of the cells in the ganglia of the sympa-
thetic trunk dispose themselves in one of the three following ways: (1) Usually
they run transversely to the long axis of the ganglion to enter a gray ramus.
344
THE NERVOUS SYSTEM
(2) The axons may run through a connecting nerve trunk into another ganglion.
He is not able to say whether these axons only run through the second ganglion
or whether they make connections with its cells. In the chick embryo he at one
time described collaterals coming from those longitudinal fibers of the ganglia,
which take origin in neighboring ganglia. Now, however, he is inclined to doubt
this observation, and thinks it likely that these collaterals all come from fibers
that have entered the sympathetic trunk through white rami at other levels.
Fig. 254.
Fig. 255.
Figs. 254 and 255. Preganglionic fibers and pericellular plexuses of the frog. Fig. 254, Pre-
ganglionic fibers, the branches of which form pericellular plexuses; Fig. 255, a unipolar sympathetic
ganglion cell in connection with which a preganglionic fiber is terminating. Methylene-blue.
(Huber.)
(3) In some cases the axons, arising from cells in the ganglia of the sympathetic
trunk, run toward the neighboring arteries in the visceral nerves.
There is no anatomic evidence worth mentioning in favor of the existence of association
neurons, uniting one sympathetic ganglion with another or one group of cells with another
within such a ganglion. But there is strong physiologic evidence against the existence of
such association neurons (Langley, 1900 and 1904) ; and Johnson (1918) has shown that none
are present in the sympathetic trunk of the frog.
Termination of the Preganglionic Fibers. The spaces among the cells of a
sympathetic ganglion are occupied by a rich intercellular plexus of dendritic
THE SYMPATHETIC NERVOUS SYSTEM 345
branches and fine axons (Figs. 251, A ; 253). The fine axons represent the rami-
fications of preganglionic fibers and they degenerate when the connection
between the ganglion and the central nervous system is severed (Ranson
and Billingsley, 1918). Similar fibers pierce the capsules surrounding the
cells and intertwine with the intracapsular dendrites. No doubt synaptic
relations are established between the axonic and dendritic ramifications in
these plexuses.
Another and very characteristic type of synapse is established in the peri-
cellular plexuses, formed by the terminal ramifications of preganglionic fibers upon
the surface of the cell bodies of postganglionic neurons. Huber (1899) showed
that fibers from the white rami branch repeatedly in the sympathetic ganglia
and that the branches terminate in subcapsular pericellular plexuses (Figs. 254,
255).
In the sympathetic ganglia of the frog the pericellular plexus seems to be the only type
of synapse and there is no intercellular plexus. In the mammalian sympathetic ganglion
these pericellular plexuses are harder to demonstrate and are probably less numerous, while
the intercellular plexus is much in evidence. It is well established that one preganglionic
fiber may be synaptically related to several postganglionic neurons, probably in some in-
stances to as many as thirty or more (Ranson and Billingsley, 1918).
COMPOSITION OF SYMPATHETIC NERVES AND PLEXUSES
Some of the sympathetic nerves are as well myelinated as the cerebrospinal
nerves and present a white glistening appearance. This is true, for example, of
the cervical portion of the sympathetic trunk, the white rami, and the splanch-
nic nerves. Such white sympathetic nerves are composed at least in large part
of fibers running to and from the central nervous system. Other nerves like
the gray rami and branches to the blood-vessels are gray, because they are com-
posed chiefly of unmyelinated postganglionic fibers. In preceding paragraphs
we have shown that there are probably no association or sensory neurons in
the sympathetic ganglia; and, if this be true, there are no axons, arising from such
cells, in the sympathetic nerve trunks and plexuses. These nerves and plexuses
are composed of the following three kinds of fibers (Fig. 256) : (1) Preganglionic
visceral efferent fibers, which are of small size and myelinated, have their cells
of origin in the cerebrospinal axis, and terminate in the sympathetic ganglia.
(2) Postganglionic fibers, which are for the most part unmyelinated, have their
cells of origin in the sympathetic ganglia and terminate in involuntary muscle or
glandular tissue. (3) Visceral afferent fibers, which include myelinated fibers
of all sizes as well as many that are unmyelinated, have their cells of origin in
346
THE NERVOUS SYSTEM
the cerebrospinal ganglia and terminate in the viscera. The statements con-
tained in this paragraph should not be applied without qualification to the ter-
Spinal ganglion
Dorsal root
Pacinian corpuscle '
Motor ending on smooth
muscle'
Ventral root
Splanchnic nerve
Collateral ganglion
Blood-vessel-s?
^- Ganglion of sympathetic trunk
Jfr^Tji Gray ramus
"" White ramus
Sympathetic trunk
Dorsal ramus
Ventral ramus
f$ Gland
'^'^^-^ Blood-vessel
*~ White ramus
_ x Gray ramus
Ganglion of sympathetic trunk
Sympathetic trunk
Sensory ending
Fig. 256. Diagram showing the composition of sympathetic nerves. Black lines, visceral
afferent fibers; unbroken red lines, preganglionic visceral efferent fibers; dotted red lines, post-
ganglionic visceral efferent fibers.
minal ganglia and plexuses, since it is probable that these contain additional
elements either in the nature of sensory neurons or of a nerve net.
ARCHITECTURE OF THE SYMPATHETIC NERVOUS SYSTEM
The sympathetic trunks are two ganglionated cords, each of which consists
of a series of more or less segmentally arranged ganglia, bound together by as-
cending and descending nerve-fibers and extending from the level of the second
cervical vertebra to the coccyx (Figs. 248, 257). The two trunks are symmetrically
placed along the anterolateral aspects of the bodies of the vertebrae. There are
21 or 22 ganglia in each chain; and of these, 3 are associated with the cervical
spinal nerves, 10 or 11 with the thoracic, 4 with the lumbar, and 4 with the sacral
spinal nerves. The sympathetic trunks are connected with each of the spinal
nerves by one or more delicate nerve strands, called rami communicantes (Figs.
THE SYMPATHETIC NERVOUS SYSTEM 347
248, 257). To each spinal nerve there runs a gray ramus from the sympathetic
trunk. The white rami, on the other hand, are more limited in distribution and
unite the thoracic and upper four lumbar nerves with the corresponding portion
of the sympathetic trunk.
The white rami consist of visceral afferent and preganglionic visceral efferent
fibers directed from the central into the sympathetic nervous system. They
contribute the great majority of the ascending and descending fibers of the
sympathetic trunk (Fig. 257). While some of the fibers may terminate in the
ganglion with which the white ramus is associated, and others run directly
through the trunk into the splanchnic nerves, the majority of the fibers turn
either upward or downward in the trunk and run for considerable distances within
it (Fig. 250). The fibers from the upper white rami run upward, those from the
lower white rami downward, while those from the intermediate rami may run
either upward or downward. The cervical portion of the sympathetic trunk
consists almost or quite exclusively of ascending fibers, the lumbar and sacral
portions of the trunk largely of descending fibers from the white rami. The
afferent fibers of the white rami merely pass through the trunk and its branches
to the viscera. The preganglionic fibers, with the exception of those which run
out through the splanchnic nerves, end in the gang^a of the trunk. Here they
enter into synaptic relations with the postganglionic neurons. The majority
of the postganglionic neurons, located in the ganglia of the sympathetic trunk,
send their axons into the gray rami (Figs. 250, 256).
The gray rami are composed of postganglionic fibers directed from the sym-
pathetic trunk into the spinal nerves. These unmyelinated fibers, after joining j
the spinal nerves, are distributed with them as vasomotor, secretory, and pilo-
motor fibers to the blood-vessels, the sweat glands, and the smooth muscle of
the hair-follicles.
Especially in the cervical region there are other important branches from the
sympathetic trunk, which resemble the gray rami in structure and which convey
postganglionic fibers to certain of the cranial nerves and to the heart, pharynx,
the internal and external carotid and thyroid arteries, and through the plexuses
on these arteries to the thyroid gland, salivary glands, eye, and other structures
(Figs. 248, 250, 257).
The cranial portion of the sympathetic trunk consists of three ganglia bound
together by ascending preganglionic fibers from the white rami. In the cat it has
been shown to contain few if any sensory or postganglionic fibers. The superior
cervical ganglion is the largest of the three ganglia and from it there are given off
548 THE NERVOUS SYSTEM
numerous gray nerve strands. These are all composed of postganglionic fibers
which arise in this ganglion. They run to the neighboring cranial and spinal
nerves, to which they carry vasomotor, pilomotor, and secretory fibers, and to the
heart, pharynx, and the internal and external carotid arteries (Figs. 248, 250,
257). The most important of these branches of the superior cervical ganglion
are the three following: (1) The superior cervical cardiac nerve, which runs
from the superior cervical ganglion to the cardiac plexus, carries accelerator
fibers to the heart. (2) The internal carotid nerve runs vertically from the
ganglion to the internal carotid artery, about which its fibers form a plexus,
known as the internal carotid plexus (Fig. 257). It is by way of this nerve and
plexus that the pupillary dilator fibers reach the eye (Fig. 247). (3) The branch
of the superior cervical ganglion to the external carotid artery breaks up into a
plexus on that artery. A continuation of this plexus extends along the external
maxillary artery, and carries secretory fibers to the submaxillary salivary gland.
The middle and inferior cervical sympathetic ganglia are smaller. Among
the branches from these ganglia we may mention the gray rami to the adjacent
spinal nerves and the middle and inferior cardiac nerves to the cardiac plexus
(Figs. 248, 257).
The thoracic portion of the sympathetic trunk is connected with the thoracic
nerves by the gray and white rami. In addition to the rami communicantes
and some small branches to the aortic and pulmonary plexuses, there are three
important branches of the thoracic portion of the sympathetic trunk known as
the splanchnic nerves. These run through the diaphragm for the innervation
of abdominal viscera (Figs. 248, 257). The greater splanchnic nerve is usually
formed by branches from the fifth to the ninth thoracic sympathetic ganglia
and after piercing the diaphragm joins the celiac ganglion. The smaller splanch-
nic nerve is usually formed by branches from the ninth and tenth thoracic
sympathetic ganglia and terminates in the celiac plexus. The lowermost splanch-
nic nerve arises from the last thoracic sympathetic ganglion and terminates in
the renal plexus. These splanchnic nerves, although they appear to be branches
of the thoracic sympathetic trunk, are at least in major part composed of fibers
from the white rami, which merely pass through the trunk on their way to the
ganglia of the celiac plexus (Figs. 250, 257; Langley, 1900; Ranson and Billings-
ley, 1918).
THE SYMPATHETIC PLEXUSES
The Sympathetic Plexuses of the Thorax. In close association with the
vagus nerve in the thorax are three important sympathetic plexuses. The
THE SYMPATHETIC NERVOUS SYSTEM 349
cardiac plexus lies in close relation to the arch of the aorta, and from it sub-
ordinate plexuses are continued along the coronary arteries. It receives the
three cardiac sympathetic nerves from the cervical portion of each sympathetic
trunk, as well as branches from both vagus nerves (Figs. 248, 257). The pregan-
glionic fibers of the vagus terminate in synaptic relation with the cells of the
cardiac ganglia. They convey inhibitory impulses which are relayed through
these ganglia to the cardiac musculature (Fig. 250). The cardiac sympathetic
nerves contain postganglionic fibers which take origin in the cervical sympa-
thetic ganglia; and they relay accelerator impulses, coming from the spinal cord
by way of the upper white rami and sympathetic trunk to the heart (Fig. 250).
The pulmonary and esophageal plexuses of the vagus are also to be regarded as
parts of the sympathetic system (Fig. 257).
The celiac plexus (solar plexus) is located in the abdomen in close relation
to the celiac artery (Figs. 248, 257). It is continuous with the plexus which
surrounds the aorta. Subordinate portions of the celiac plexus accompany
the branches of the celiac artery and the branches from the upper part of the
abdominal aorta. These are designated as the phrenic, suprarenal, renal,
spermatic or ovarian, abdominal aortic, superior gastric, inferior gastric, he-
patic, splenic, superior mesenteric, and inferior mesenteric plexuses. The celiac
plexus contains a number of ganglia which in man are grouped into two large
flat masses, placed one on either side of the celiac artery and known as the
celiac ganglia. These ganglia are bound together by strands which cross the
median plane above and below this artery. Somewhat detached portions of
the celiac ganglion, which lie near the origin of the renal and superior mesenteric
arteries, are known respectively as the aorticorenal and superior mesenteric
ganglia. In addition, there is a small mass of nerve-cells in the inferior mesen-
teric plexus close to the beginning of the inferior mesenteric artery. This is
known as the inferior mesenteric ganglion.
Preganglionic fibers reach the celiac plexus from two sources, namely, from
the white rami by way of the sympathetic trunk and splanchnic nerves and from
the vagus nerve (Fig. 257). Most if not all of the preganglionic fibers contained
in the splanchnic nerves terminate in the ganglia of the celiac plexus. At the
lower end of the esophageal plexus the fibers from the right vagus nerve become
assembled into a trunk which passes to the posterior surface of the stomach and
the celiac plexus. The fibers of the left vagus pass to the anterior surface of
the stomach and to the hepatic plexus (Fig. 257). It is probable that the pre-
ganglionic fibers of the vagus do not terminate in the ganglia of the celiac plexus,
35
THE NERVOUS SYSTEM
N. VII
Internal carotid plexus
ToN.X
ToN.IX
To cervical N. I
To sacral N. I
N.II
N.III
IV
Visceral branches
sacral nerves
N.IV
N. V
To coccygeal nerve
Ciliary ganglion
Splenopalatine ganglion
N. IX
Otic ganglion
Superior cervical ganglion
Pharyngeal plexus
N, VII
Submaxillary ganglion
Middle cervical ganglion
Superior cardiac N.
Middle cardiac N.
Inferior cardiac N.
Cardiac branches of vagus
Vagus and left pulmonary plexus
Cardiac plexus
Left coronary plexus
Esophageal plexus
Splanchnic nerves
Hepatic plexus
Left vagus nerve
^Gastric plexus
.Myenteric and sub-
mucous plexuses
Splenic plexus
Celiac plexus
perior mesenteric
plexus
^ *
Inferior mesenteric plexus
^Abdominal aortic plexus
4
Hypogastric plexus
Fig. 257. Diagram of the sympathetic nervous system. The red lines indicate the branches
of the cerebrospinal nerves which join the sympathetic system and those sympathetic nerves which
are composed in major part of fibers from the cerebrospinal nerves. (Modified from Jackson-
Morris.)
but merely pass through that plexus to end in the terminal ganglia, such as the
small groups of nerve-cells in the myenteric and submucous plexuses of the in-
testine (Fig. 250).
THE SYMPATHETIC NERVOUS SYSTEM 351
The my enteric plexus (of Auerbach) and the submucous plexus (of Meissner),
located within the walls of the stomach and intestines, receive filaments from
the gastric and mesenteric divisions of the celiac plexus. They also receive
fibers from the vagus either directly, as in the case of the stomach, or indirectly
through the celiac plexus (Fig. 257). Unfortunately, very little is known con-
cerning the synaptic relations established in the ganglia of these plexuses. Ac-
cording to Langley, the postganglionic fibers from the celiac ganglia run through
these plexuses without interruption and end in the muscular coats and glands
of the gastro-intestinal tract. The preganglionic fibers from the vagus probably
end in synaptic relation to cells in these small ganglia; and the axons of these
cells serve as postganglionic fibers, relaying the impulses from the vagus to the
glands and muscular tissue. As was indicated in a preceding paragraph, the
enteric plexuses must also contain a mechanism for purely local reactions, since
peristalsis can be set up by distention in an excised portion of the gut. But
as yet we are entirely ignorant as to what that mechanism may be.
The hypogastric plexus is formed by strands which run into the pelvis from
the lower end of the aortic plexus and are joined by the visceral branches of the
second, third, and fourth sacral nerves and by branches from the sympathetic
trunk (Figs. 248, 257). As the hypogastric plexus enters the pelvis it splits into
two parts, which lie on either side of the rectum and are sometimes called the
pelvic plexuses. From these plexuses branches are supplied to the pelvic vis-
cera and the external genitalia.
The Cephalic Ganglionated Plexus. In close topographic relation to the
branches of the fifth cranial nerve are four sympathetic ganglia, known as the
ciliary, sphenopalatine, otic, and submaxillary ganglia. Each of these is con-
nected with the superior cervical sympathetic ganglion by filaments derived
from the plexuses on the internal and external carotid arteries and their branches
(Fig. 257). These filaments are designated in descriptive anatomy as the sym-
pathetic roots of the ganglia. Each ganglion receives preganglionic fibers from
one of the cranial nerves by way of what is usually designated as its motor root
(Fig. 257). Thus the ciliary ganglion receives fibers from the oculomotor nerve;
the sphenopalatine ganglion receives fibers from the facial nerve by way of the
great superficial petrosal nerve and the nerve of the pterygoid canal; the otic
ganglion receives fibers from the glossopharyngeal nerve (Miiller and Dahl, 1910) ;
and the submaxillary ganglion receives fibers from the facial nerve by way of
the nervus intermedius and the lingual nerve. Postganglionic fibers arising
in these ganglia are distributed to the structures of the head. From the ciliary
352 THE NERVOUS SYSTEM
ganglion fibers go to the intrinsic musculature of the eye. Some of the fibers
arising in the sphenopalatine ganglion go to the blood-vessels in the mucous
membrane of the nose. Fibers from the otic ganglion reach the parotid gland.
And those arising in the submaxillary ganglion end in the submaxillary and
sublingual salivary glands (Fig. 250).
IMPORTANT CONDUCTION PATHS BELONGING TO THE AUTONOMIC NERVOUS
SYSTEM
Thanks to the work of Langley, we know that the conduction pathways in
the sympathetic nervous system are at least as sharply defined as those in the
brain and spinal cord. A great deal has already been done in the way of tracing
these pathways; and some of the more important of these are given in the out-
line which follows:
1. Paths for the efferent innervation of the eye (Figs. 247, 250):
(a) Ocular craniosacral pathway.
Preganglionic neurons: Cells in the Edinger-Westphal nucleus,
fibers by way of the third cranial nerve to end in the ciliary ganglion.
Postganglionic neurons: Cells in the ciliary ganglion, fibers by
way of the short ciliary nerves to the ciliary muscle and the circular
fibers of the iris.
Function: Accommodation and contraction of the pupil.
(b) Ocular thoracicolumbar pathway.
Preganglionic neurons: Cells in the intermediolateral column of
the spinal cord, fibers by way of the upper white rami and sympathetic
trunk to end in the superior cervical ganglion.
Postganglionic neurons: Cells in the superior cervical ganglion,
fibers by way of the internal carotid plexus to the ophthalmic division
of the fifth nerve, the nasociliary and long ciliary nerves of the eyeball;
other fibers pass from the internal carotid plexus through the ciliary
ganglion, without interruption, into the short ciliary nerves and to
the eyeball.
Function: Dilatation of the pupil by the radial muscle-fibers of
the iris.
2. Paths for the efferent innervation of the submaxillary gland (Fig. 250) :
(a) Submaxillary craniosacral pathway.
Preganglionic neurons: Cells in the nucleus salivatorius superior,
fibers by way of the seventh cranial nerve, chorda tympani, and
THE SYMPATHETIC NERVOUS SYSTEM 353
lingual nerve to end in the portion of the submaxillary ganglion
located on the submaxillary duct.
Postganglionic neurons: Cells in a number of groups along the
chorda tympani fibers as they follow the submaxillary duct, fibers
distributed in branches to the submaxillary gland.
Function : Increases secretion.
(&) Submaxillary thoracicolumbar pathway.
Preganglionic neurons: Cells in the intermediolateral column of
the spinal cord, fibers by way of the upper white rami, and the sym-
pathetic trunk to end in the superior cervical ganglion.
Postganglionic neurons: Cells in the superior cervical ganglion,
fibers by way of the plexuses on the external carotid and external
maxillary arteries to the submaxillary gland.
Function: Increases secretion.
3. Paths for the efferent inner vation of the heart:
(a) Cardiac craniosacral pathway.
Preganglionic neurons: Cells in the dorsal motor nucleus of the
vagus, fibers through the vagus nerve to the intrinsic ganglia of the
heart, in which they end.
Postganglionic neurons: Cells in the intrinsic cardiac ganglia,
fibers to the cardiac muscle.
Function: Cardiac inhibition.
(&) Cardiac thoracicolumbar pathway.
Preganglionic neurons: Cells in the intermediolateral column of
the spinal cord, fibers by way of the upper white rami and the sym-
pathetic trunk to the superior, middle, and inferior cervical ganglia.
Postganglionic neurons: Cells in the cervical ganglia of the sym-
pathetic trunk, fibers by way of the corresponding cardiac nerves to
the musculature of the heart.
Function: Cardiac acceleration.
4. Paths for the efferent innervation of the musculature of the stomach
exclusive of the sphincters (Fig. 250) :
(a) Gastric craniosacral pathway.
Preganglionic neurons: Cells in the dorsal motor nucleus of the
vagus, fibers by way of the vagus nerve, to end in the intrinsic ganglia
of the stomach.
THE NERVOUS SYSTEM
Postganglionic neurons: Cells in the intrinsic gastric ganglia, fibers
to end in the gastric musculature.
Function: Excites peristalsis.
(6) Gastric thoracicolumbar pathway.
Preganglionic neurons: Cells in the intermediolateral column of the
spinal cord, fibers by way of the white rami from the fifth or sixth to
the twelfth thoracic nerves, through the sympathetic trunk without
interruption, and along the splanchnic nerves to the celiac ganglion,
where they end.
Postganglionic neurons: Cells in the celiac ganglion, fibers by way
of the celiac plexus and its offshoots to the stomach, to end in the
musculature of the stomach.
Function: Inhibits peristalsis.
5. Paths for the efferent innervation of the musculature of the urinary
bladder,
(a) Vesical craniosacral pathway.
Preganglionic neurons: Cells in the lateral part of the anterior
gray column in the sacral portion of the spinal cord, fibers by way
of the second and third sacral nerves and their visceral rami through
the pelvic plexus to the plexus upon the wall of the bladder.
Postganglionic neurons: Cells in the small ganglia of the vesical
plexus, fibers to the vesical musculature.
Function: Excites contraction of the vesical musculature exclusive
of the internal sphincter (trigonal area), the contraction of which it
inhibits and thus produces urination.
(6) Vesical thoracicolumbar pathway.
Preganglionic neurons : Cells in the caudal part of the intermedio-
lateral cell column, fibers by way of the lower white rami to the infe-
rior mesenteric ganglion.
Postganglionic neurons: Cells in the inferior mesenteric ganglion,
fibers through the inferior mesenteric plexus to the musculature of
the bladder.
Function: Excites contraction of the internal sphincter (trigonal
area of the vesical musculature), causing retention of urine.
It will be noted that the viscera receive a double autonomic innervation, and
that the impulses transmitted along the craniosacral pathways are usually
antagonistic to those transmitted along the thoracicolumbar paths.
A LABORATORY OUTLINE OF NEURO-ANATOMY
THE following directions for the study of the gross and microscopic anatomy of
the nervous system are intended to aid the student in making the best use of his time
and laboratory material. Free use is made of the sheep's brain because in most in-
stitutions the number of human brains available is limited, and these are often poorly
preserved and entirely unsuited for dissection. Even if an unlimited supply of well-
preserved human brains were at hand, there would still be an advantage in the use of
the sheep's brain because in it certain structures (such as the olfactory tracts and centers
and the really significant subdivisions of the cerebellum) are more easily seen and more
readily understood.
The outline has been written in such a way that it can be readily adapted by the
instructor to meet his own needs. It is assumed that each instructor will furnish his
students with a schedule for the laboratory work, showing the number of laboratory
periods available and the topics to be covered each period. This will help the student
properly to apportion his time and enable the instructor to arrange the order of the
laboratory work to his own liking. The paragraphs have been numbered serially in
order that in such a schedule they may be referred to by number. It is not necessary
that the topics be taken up in their numeric order. And in a course of one hundred
hours some of the topics should be omitted altogether. How much should be omitted
will depend largely on the amount of drawing required. It is assumed that the in-
structor will indicate on the laboratory schedule the drawings which he wishes to have
made. For this reason we have, for the most part, omitted specific directions for draw-
ings.
Since it will be necessary for the student in using the outline to make frequent
references to figures in the text, it will be convenient to keep in the book several strips
of thin paper to serve as bookmarks.
METHODS OF BRAIN DISSECTION
Much information concerning the gray masses and fiber tracts of the brain can be
obtained by dissection. This should be carried out, for the most part, with blunt
instruments. It is rarely necessary to make a cut with a knife. An orangewood mani-
cure stick makes an excellent instrument. It should be rounded to a point at one end
for teasing, while the larger end should be adapted for scraping away nuclear masses.
A pair of blunt tissue forceps of medium size with smooth even edges and fine transverse
interlocking ridges is also an essential instrument. This is useful in grasping and strip-
ping away small bundles of fibers. In dissecting out a fiber tract it is necessary to have
in mind a clear idea of the position and course of the tract, and the dissecting instru-
ments should be carried in the direction of the fibers. Where it is necessary to remove
nuclear material in order to display fiber bundles, it will be found very helpful to let a
stream of water run over the specimen while the dissection is in progress.
355
356 THE NERVOUS SYSTEM
DISSECTION OF THE HEAD OF THE DOGFISH
1. The dogfish is the smallest of the sharks. Either the spiny dogfish (Squalus
acanthias) or the smooth dogfish (Mustelus canis) may be used for dissection.
2. The special sense organs include the olfactory organs, the eyes, the ears, and
certain sense organs in the skin, known as the lateral line canals, and the ampullae of
Lorenzini.
3. Locate the position of the lateral line canal which produces a light colored ridge
in the skin extending from head to tail along either side of the body. The line may be
recognized by the presence of numerous small pores which open into the canal. It
extends on to the head and there forms the supraorbital, infraorbital, and hyoman-
dibular canals. The ampulla of Lorenzini are bulb-shaped bodies connected by long
canals with pores in the skin. They are irregularly arranged and are most numerous
on the snout.
4. Locate the olfactory organs or nasal capsules which have their openings on the
ventral surface of the snout in front of the mouth.
5. Note the gills and spiracles (Fig. 12). Find two minute apertures near the
midline between the spiracles. These are the openings of the endolymphatic ducts.
6. The internal ear, a membranous labyrinth inclosed in a cartilaginous capsule,
should be exposed on the left side. Shave off the cartilage in thin slices in the region
between the spiracle and the median plane. The membranous labyrinth can be seen
through the translucent cartilage, and care should be exercised to avoid injuring it while
the cartilage is being removed. It consists of a spheric sac, the utriculosaccular chamber,
to which there are attached three semicircular canals (Fig. 12). The endolymphatic
duct is a small canal, which extends from this chamber through the roof of the skull to
the small opening in the skin, which has previously been identified. Note the enlarge-
ment at one end of each semicircular canal, known as the ampulla, and observe that
each of these canals lies in a plane at right angles to the planes of the other two.
7. The Brain and Cranial Nerves. Remove the remainder of the roof of the skull
and expose the brain, eyes, and cranial nerves.
8. Examine the brain as seen from the dorsal surface. Note the continuity of the
medulla oblongata with the spinal cord. Identify the cerebellum, the thalamus, epiphysis,
habenula, cerebral hemispheres, and olfactory bulbs (Fig. 8 and pp. 26-31).
9. By dissection display on the left side the eye-muscles and the nerves which in-
nervate them, as well as the optic nerve (Fig. 12).
10. Find the nervus terminalis (Fig. 8). Now locate each of the cranial nerves
from the second to the tenth inclusive, and trace them from the brain as far as possible
toward their peripheral terminations (Figs. 12, 13). Note particularly that Nn. VII
and X each have an extra root, indicated in black in Fig 13, which carries fibers from
the lateral line organs to the acusticolateral area of the medulla.
11. Attention should now be paid to the functional types of nerve-fibers which
compose each of the cranial nerves (see pp. 168-170 and Figs. 119, 120). The ac-
companying table shows in which of the cranial nerves of the dogfish each of the four
principal functional groups of fibers are to be found (Herrick and Crosby, 1918).
A LABORATORY OUTLINE OF NEURO-ANATOMY
CRANIAL NERVE COMPONENTS OF THE DOGFISH
357
Somatic sensory.
Somatic motor.
Visceral sensory.
Visceral motor.
II. Optic
III. Muscle sense
IV. Muscle sense
V. General cutaneous
VI. Muscle sense
VII. Lateral line fibers
VIII. To the ear
IX. Lateral line fibers
X. Lateral line and
general cutaneous
fibers
III. To eye-muscles
IV. To eye-muscles
VI. To eye-muscles
I. Olfactory
VII. General visceral
and gustatory
IX, X. General visceral
and gustatory
III. For intrinsic muscles
of the eye
V. To the jaw muscles
VII. To hyoid muscula-
ture
IX, X. To branchial and
general visceral mus-
culature
12. There are six pairs of cranial nerves associated with the medulla oblongata. The
tenth cranial or vagus nerve is one of the largest and arises by two series of roots. One
group of rootlets springs from the dorsolateral aspect of the medulla oblongata near its
lower end, and contains fibers which are distributed through the branchial and gastro-
intestinal rami of the vagus, while a large root, carrying fibers for the lateral line sense
organs, runs farther cephalad and enters the acusticolateral area. The ninth or glosso-
pharyngeal nerve, the nerve of the first branchial arch, arises from the medulla ob-
longata just ventral to this root of the vagus. Since the gills, as well as the gastro-
intestinal tract, are visceral organs, both the ninth and tenth nerves carry many visceral
fibers. The eighth or acoustic nerve arises from the side of the medulla opposite the
caudal part of the cerebellum in company with the fifth and seventh nerves, and ends
in the membranous labyrinth of the ear. Like the vagus, the facial or seventh cranial
nerve has, in addition to its main root, another, which runs further dorsally into the
acusticolateral area. This root carries sensory fibers for the lateral line organs of the
head. The sixth or abducens nerve arises more ventrally at the same level as the eighth^
The fifth, or trigeminal nerve, which sends many branches to the skin of the head, is
represented by a large root emerging from the medulla oblongata in company with
the seventh. Some idea of the peripheral distribution of these nerves can be gained
from a study of Figs. 12 and 13.
13. The floor of the fourth ventricle should now be exposed by carefully tearing away
the membranous roof of that cavity. The floor presents for examination a series of
longitudinal ridges and furrows which are of importance because they mark the position
of longitudianl columns (Figs. 8, 13), to each of which a special functon can be assigned.
A ridge on either side of the midline represents the position of the median longitudinal
bundle, beneath which lie the nuclei of the third, fourth, and sixth cranial nerves.
Since these nerves supply somatic musculature, the longitudinal elevation marks
the position of the somatic motor column. Separated from this ridge by a broad furrow
258 THE NERVOUS SYSTEM
is a more prominent ridge with tooth-like secondary elevations. Within this second
ridge terminate the fibers of visceral sensation and taste from the seventh, ninth, and
tenth nerves. It is known as the visceral lobe or -visceral sensory column. Beneath the
groove which separates these two ridges are located the motor nuclei of the fifth,
seventh, ninth, and tenth cranial nerves. These nuclei supply visceral musculature
and constitute the visceral motor column. The dorsal part of the lateral wall of the fossa
forms another prominent ridge, which just caudal to the cerebellum is redundant and
folded on itself to form an ear-shaped projection. This auricular fold, sometimes
called the lobus linese lateralis, and the prominent margin just caudal to it belong to the
acusticolateral area and contain the centers for the reception of impulses coming from
the ear (N. VIII) and from the sense organs of the lateral line (Nn. VII and X). Ad-
jacent to the acusticolateral area is a portion of the medulla oblongata which is concerned
with the reception of sensory impulses from the skin which reach the medulla oblongata
along the fifth and tenth nerves. The nuclei of the acusticolateral and general cutane-
ous areas together constitute the somatic afferent column.
14. Locate these functional columns on your specimen. Note the close relation of
the olfactory bulb to the nasal sac. By comparison with Fig. 13 locate on your speci-
men the olfactory portions of the brain. What part of the brain is especially associated
with the eyes?
15. Cut the nerve roots at some distance from the brain. Remove the brain,
being careful not to injure the olfactory bulbs. Now study the lateral and ventral
surfaces of the brain in order to locate more accurately the points of origin of the various
cranial nerves (Fig. 10).
16. Now study the parts of the brain which belong to the rhombencephalon. Which
parts are they, and what is their relationship to each other? (Figs. 8, 10 and p. 26.)
17. Study the parts of the brain which belong to the mesencephalon. Which
are they, and what relationship do they bear to each other? (Figs. 8, 10 and
p. 28.)
18. In the same way study the parts belonging to the diencephalon (Figs. 8, 10
and pp. 28, 29). Make a list of these parts. Tear away the membranous roof of the
third ventricle and examine that cavity.
19. Note the external form of the telencephalon and the parts which compose it
(Figs. 8, 10). Students working at adjacent* tables should cooperate in the work
which follows in order that two sharks' brains may be available. With a sharp razor
blade divide one in the medial sagittal plane; and with a sharp scalpel open up the
ventricles in the other as indicated in Fig. 9. Study the ventricles of the brain as they
are displayed in these preparations and in Figs. 9 and 11.
20. Find the velum transversum and the ridge produced by the optic chiasma.
All that part of the brain which lies rostral to these structures belongs to the telen-
cephalon. Study the telencephalon in detail (Figs. 8-11 and p. 30). Of what parts
is it composed, and what are their relations, to each other? Pay special attention to
the several parts of the telencephalic cavity.
THE BRAIN OF THE FETAL PIG
21. Using a pig embryo of about 35 mm., slice off the skin and a small amount of
the underlying tissue on either side of the head with a sharp razor. Then at one careful
A LABORATORY OUTLINE OF NEURO-ANATOMY
359
stroke split the specimen lengthwise in the median plane. This provides two prepara-
tions for dissection, which should be used by two students.
Pineal body
Third ventricle
Hypothalamus
Thalamus
Chorioid plexus of lateral
ventricle
Lateral ventricle
Corpus striatum
Lamina terminalis
Rhinencephalon
Hypophysis
Tongue
Fig. 258. Medial sagittal section of the head of a 35 mm. pig embryo. (Redrawn from Prentiss-
Arey.)
22. First study the medial section of the brain, noting the five divisions of the
brain, the ventricles, and the relation of the cerebral hemispheres to other parts of the
Cerebral aqueduct
Lamina quadrigemina
Cerebral peduncle
Cerebellum
Chorioid plexus of fourth ventricle
Fourth ventricle
Medulla oblongata
Central canal of spinal
cord
Semilunar ganglion N. V
Mesencephalon
Cerebellum
Hypothalamus
Geniculate gang. N. VII
Ganglion N. VIII
Medulla oblongata
Jugular gang. N. X
Gang, of Froriep
Gang. N. cerv. I
Accessory nerve
Hypoglossal nerve
Ganglion nodosum N. X
Gang. N. cerv. V
Cerebral hemisphere
N. V, ophthalmic N.
Rhinencephalon
N. opticus
N. V, maxillary N.
N. V, mandibular N.
Chorda tympam
Facial N.
Fig. 259. Dissection of the head of a 35 mm. pig embryo. Lateral view. (Redrawn from
Prentiss-Arey.)
brain (Fig. 258. See also Figs. 16, 17 and pp. 32-36). Of what three parts is the
cerebral hemisphere composed? Locate each of the subdivisions of the diencephalon.
360 THE NERVOUS SYSTEM
To which part does the pineal body belong? The hypophysis? Locate the quadri-
geminal lamina, cerebral peduncle, cerebellum, and medulla oblongata.
23. Now turn the specimen over and carefully dissect away what remains of the
skin and mesodermal tissues so as to expose the brain and cranial nerves from the lateral
side. Identify all the parts labeled in Fig. 259.
GENERAL TOPOGRAPHY OF THE BRAIN
24. The adult mammalian brain should now be compared with that of the shark
and with that of the fetal pig. If two sheeps' brains are available, one should be divided
into lateral halves by a cut made exactly 1 mm. to the left of the median sagittal plane.
Use a long, thin brain knife and make the cut with a single sweep. Put away the right
half for future study. On the left half and on the intact brain identify all of the chief
divisions of the brain, determine their embryologic derivation, and compare them
with similar parts in the brains of the shark and fetal pig. (See the table on p. 36,
pp. 113-116, and Figs. 82-84.)
25. By a study of the medial aspect of the left half of the brain ascertain what
relations the various subdivisions bear to each other. (See Fig. 84 and pp. 116-118.)
Note the difference in color between the cortex and the white center of the cerebellum.
By tearing away the cerebellum a little at a time make a dissection of the cerebellar
peduncles on this half of the brain (Figs. 87, 91). Scrape away the superficial gray
matter from the rostral end of the left hemisphere and uncover the white substance
beneath. The superficial gray matter is known as the cerebral cortex and this covers
the white center of the cerebral hemisphere.
NEUROLOGIC STAINS
26. Some knowledge of how various stains act on the nervous tissues is essential
for an understanding of the special preparations which are to be studied. The technic
involved in preparing such material is described in books devoted to technical methods
(Hardesty, 1902; Guyer, 1917).
27. Osmic Acid. Small nerves may be fixed in osmic acid. This stains the myelin
sheaths black. Why? Axons remain unstained.
28. The Weigert or Pal-Weigert Method. When a portion of the brain or spinal
cord has been treated for several \veeks with a solution containing potassium bichromate
(Miiller's fluid) the myelin sheaths acquire a special affinity for hematoxylin, by
virtue of which they become deep blue in color when stained by this method. Axons,
nerve-cells, and all other tissue elements remain colorless unless the preparation has
been counterstained. The method is adapted for the study of the development and
extent of myelination and for tracing myelinated fiber tracts. This method may also
be used for a study of degenerated fiber tracts, which remain colorless in preparations
in which the normal fiber tracts are well stained.
29. The Marchi method is a differential stain for degenerating fibers. These
contain droplets of chemically altered myelin. The tissue is fixed in a solution contain-
ing potassium bichromate (Muller's fluid). This treatment prevents the normal
myelinated fibers from staining with osmic acid, but does not prevent the droplets of
chemically altered myelin in the degenerated fiber from being stained black by this
A LABORATORY OUTLINE OF NEURO-ANATOMY 361
reagent. In a section prepared by this method the normal myelinated fibers are light
yellow, while the degenerated fibers are represented by rows of black dots.
30. The newer silver stains, including the Cajal method and the pyridin-silver technic,
depend upon the special affinity for silver nitrate possessed by nerve-cells and their
processes. After treatment with silver nitrate the tissue is transferred to a solution
of pyrogallic acid or hydroquinon which reduces the silver in the neurons to a metallic
state. Nerve-cells and their processes are stained yellow or brown by these methods.
Myelin sheaths remain unstained. The axis-cylinders of the myelinated fibers are
light yellow, the unmyelinated axons are dark brown or black. The neurofibrils are
stained somewhat more darkly than other parts of the cytoplasm.
31. The Golgi method furnishes preparations which demonstrate the external
form of the neurons, and make it possible to trace individual axons and dendrites for
considerable distances. The method also stains neuroglia. It is selective and rather
uncertain in its results, since only a small proportion of the nerve-cells are impregnated
in any preparation. The stain is due to the impregnation of the nerve-cells and their
processes with silver.
32. The best stains for demonstrating the tigroid masses or Nissl bodies are
toluidin blue and Nissl' s methylene-blue. Both are basic dyes; and in properly fixed
nervous tissue they color the tigroid masses as well as the nuclear chromatin of nerve-
cells blue.
THE PERIPHERAL NERVOUS SYSTEM
33. The Spinal Ganglia. Study a longitudinal section through a spinal nerve and
its roots, including the spinal ganglion, stained by the pyridin-silver method. How
are myelinated and unmyelinated axons stained by this method? What kinds of cells
do you find? Study their axons. (See Figs. 39, 40 and pp. 62-66.) Look for the
bifurcation of the myelinated and unmyelinated fibers. Note the differences in
composition of the ventral and dorsal roots. What becomes of the various kinds of
fibers when traced peripherally? When traced toward the spinal cord? What is the
origin of the unmyelinated fibers?
34. Study the vagus nerve of the dog in osmic acid and pyridin-silver preparations.
How are the various kinds of nerve-fibers stained in each? How does the structure of
the vagus differ from that of a spinal nerve?
35. Study the cervical portion of the sympathetic trunk, which in the dog lies in a
common sheath with the vagus. Of what kind of fibers is it composed? What is the
origin and termination of these fibers? (See pp. 345-347.)
36. Study the pyridin-silver preparation from the superior cervical sympathetic
ganglion. What is the source of the fine black fibers, and where do they end? Study
the ganglion cells. What becomes of their axons? (See Figs. 251, 253 and pp. 341-344.)
THE SPINAL CORD
37. Review the development and gross anatomy of the spinal cord (p. 42 and pp.
73-78). Examine the demonstration preparations of the vertebral column, showing
the spinal cord exposed from the dorsal side. In these preparations study the meninges
and ligamentum denticulatum, as well as the shape and size of the spinal cord. Note
362 THE NERVOUS SYSTEM
the level of the termination of the spinal cord, the level of the origin of the various
nerve roots and of their exit from the vertebral canal, and the level of the various seg-
ments of the cord with reference to the vertebrae. Note the filum terminale and the
cauda equina. From your text-books of anatomy study the meninges and blood-
supply of the cord.
38. The Spinal Cord in Section. Examine the Pal-Weigert sections of the cervical,
thoracic, lumbar, and sacral regions, and from them reconstruct a mental picture of the
topography of the entire cord. How does it vary in shape and size at the different
levels? Identify all the fissures, sulci, septa, funiculi, gray columns, commissures and
nerve roots, the reticular formation, the substantia gelatinosa and the caput, cervix,
and apex of the posterior gray column. (See pp. 78-84.)
39. The Microscopic Anatomy of the Spinal Cor d Study all of the histologic
preparations of the spinal cord which have been furnished you. (See pp. 85-90.)
Study the neuroglia in Golgi preparations. Study the pia mater, septa, blood-vessels,
and ependyma in hematoxylin and eosin preparations. Study the nerve-cells in Nissl,
Golgi, and silver preparations. Study the myelinated fibers in Weigert preparations
and both the myelinated and unmyelinated fibers in the silver preparations. Note
the arrangement of each of these histologic elements and be sure that you understand
the relations which they bear to each other.
40. Draw in outline, ventral side down, each of four Pal-Weigert sections taken,
respectively, through the cervical, thoracic, lumbar, and sacral regions of the human
spinal cord. Make the outlines very accurate in shape and size, with an enlargement
of 8 times. Put in the outline of the gray columns, the central canal, and the substantia
gelatinosa Rolandi. Put each outline on a separate sheet and do not ink the drawings
at present.
41. Identify the various cell columns in the gray matter and note how they vary
in the different levels of the cord (Nissl or counterstained Weigert preparations).
(See pp. 89, 90 and Fig. 65.) Indicate these cell groups in their proper places in the
four outline sketches of the spinal cord. What becomes of the axons arising from
each group of cells? Why are the anterolateral and posterolateral cell groups seen
only in the regions associated with the brachial and lumbosacral plexuses? The
intermediolateral column only in the thoracic and highest lumbar segments? Why is
the gray matter most abundant in the region of the intumescentiae and the white matter
most abundant at the upper end of the spinal cord?
42. What elements are concerned in spinal reflexes? (See pp. 91-94.)
43. What connections do the fibers of the spinal nerves establish in the spinal cord?
What is the origin and the peripheral termination of the somatic efferent fibers, of the
visceral efferent fibers, of the somatic afferent fibers, and of the visceral afferent fibers
of the spinal nerves? (See pp. 60-63 and Fig. 37.) What are the proprioceptive
and exteroceptive fibers, and in what peripheral structures do they end? (See pp.
66-72.)
44. In a pyridin-silver preparation of the cervical spinal cord of a cat note that as
the dorsal root enters the cord the unmyelinated fibers run through the lateral division
of the root into the dorsolateral fasciculus (Fig. 72). The medial division of the root
is formed of myelinated fibers which enter the posterior funiculus. Read about the
intramedullay course of these fibers (pp. 95-98).,
A LABORATORY OUTLINE OF NEURO-ANATOMY 363
45. The fiber tracts, of which the white substance is composed, cannot be distin-
guished in the normal adult cord. They can be recognized from differences in the degree
of their myelination in fetal cords (p. 112 and Fig. 79) and in preparations showing
degeneration resulting from disease or injury in various parts of the nervous system
(p. 105; Figs. 75, 76). From such preparations as are available for this purpose and
from your reading (pp. 95-112) form a clear conception of the origin, course, and ter-
mination of each of the fiber tracts.
46. Indicate the location of each of these tracts in the outline drawing of the
cervical portion of the spinal cord, entering the ascending tracts and the ventral cortico-
spinal tract on the right side, and all of the descending tracts except the ventral cortico-
spinal tract on the left side. Why should the ventral and lateral corticospinal tracts
be indicated on opposite sides of the cord? Wax crayons should be used to give the
several tracts a differential coloring. Use the following color scheme:
Somatic afferent tracts:
Proprioceptive yellow.
Exteroceptive blue.
Somatic motor tracts:
Corticospinal tracts red.
Rubrospinal tract brown.
All other tracts black.
47. The fasciculus cuneatus and fasciculus gracilis should be colored yellow and
then dotted over with blue to indicate that while the proprioceptive fibers predominate,
there are also some exteroceptive fibers in these tracts.
THE BRAIN STEM
48. Now take the human brain and identify all of its principal divisions. Dissect
out the arterial circle of Willis, and identify the branches of the internal carotid, ver-
tebral, and basilar arteries. Read about the blood-supply and meninges of the brain
in your text-book of anatomy. Identify all of the cranial nerves (Fig. 86).
49. Examine again the cerebellar peduncles in the three specimens of the sheep's
brain (Figs. 87, 91). Now remove the cerebellum from the previously intact sheep's
brain. Cut through the peduncles on both sides of the brain as far as possible from
the pons and medulla, sacrificing the cerebellum to some extent in order to leave as
much of the peduncles as possible attached to the brain stem. Be careful not to damage
the anterior medullary velum and the tela chorioidea which lie under cover of the
cerebellum (Fig. 84). In the same way remove the cerebellum from the human brain.
50. Study the roof of the fourth ventricle in both the human and the sheep's brain
(pp. 128, 129 and Figs. 84, 90, 154). Examine the chorioid plexus of the fourth ven-
tricle. Note the line of attachment of the tela chorioidea. Tear this membrane away.
The torn edge which remains attached to the medulla is the taenia of the fourth ventricle
(Figs. 89, 90). Study the attachments of the anterior medullary velum. The decus-
sation of the trochlear nerve within the velum can easily be seen in the sheep. Remove
this membrane. The floor of the fourth ventricle is now fully exposed.
51. Remove the pia mater from the brain stem, carefully cutting around the roots
of the cranial nerves with a sharp-pointed knife to prevent these nerves being torn
away from the brain when this membrane is removed.
364
THE NERVOUS SYSTEM
52. Carefully examine the medulla, pans, floor of the fourth ventricle, and the mesen-
cephalon, observing all the details mentioned on pp. 118-131 and illustrated in Figs.
84, 86-89, 91.
53. Take selected transverse sections through the human brain stem and, by com-
parison with the gross specimen, determine the level of each section.
54. Draw in outline each of these transverse sections through the brain stem.
Put each drawing on a separate page, ventral side down, with the transverse diameter
corresponding to the longer dimension of the paper. Study each preparation in detail
and identify all of the parts, indicating them lightly in pencil. Do not label the draw-
ings at this time. Make sure that all proportions are correct. The sections through
the medulla should be enlarged eight diameters, those through the pons and mesen-
cephalon four diameters.
55. Section Through the Decussation of the Pyramids. Keep in mind the tracts
which extend into the brain from the spinal cord and note the changes in their form
and position. Identify the decussation of the pyramids, the nucleus gracilis and nucleus
cuneatus, the spinal root of the trigeminal nerve and its nucleus, the reticular formation.
Note the change in the form of the gray substance (pp. 132-137; Figs. 94, 95, 98).
56. Section Through the Decussation of the Lemniscus. Note the rapid change in
the form of the gray matter. Identify the internal and external arcuate fibers, the
decussation of the lemniscus and the beginning of the medial lemniscus, as well as the
structures continued up from the preceding level (Figs. 96, 99; pp. 137-139).
57. Section Through the Olive and the Hypoglossal Nucleus. At this level the central
canal opens out into the fourth ventricle. The posterior funiculi and their nuclei are
disappearing or have disappeared. The dorsal spinocerebellar tract lies lateral to the
spinal tract of the trigeminal nerve and is directed obliquely backward toward the
restiform body. Identify, in addition to those structures which are continued from
the preceding level, the inferior olivary nucleus with the olivocerebellar fibers, the
dorsal and medial accessory olivary nuclei, the external arcuate fibers, the nucleus and
fibers of the hypoglossal nerve, the dorsal motor nucleus of the vagus, the tractus
solitarius and its nucleus, the nucleus ambiguus and the lateral reticular nucleus (Figs.
97,101; pp. 139-142).
58. Section Through the Restiform Body. The restiform body and the spinal tract
of the fifth nerve are conspicuous in the dorsolateral part of the section. In the floor
of the fourth ventricle locate the nucleus of the hypoglossal nerve, the dorsal motor
nucleus of the vagus, the medial and the spinal vestibular nuclei. The spinal tract of
the fifth nerve and its nucleus are deeply situated ventral to the restiform body and
broken up by the olivocerebellar fibers (Fig. 103; pp. 143-146).
59. Section Through the Lower Margin of the Pons. Identify such portions of the
pons, brachium pontis, and cerebellum as are contained in the section. Dorsolateral
to the restiform body is the dorsal cochlear nucleus, and ventrolateral to it the ventral
cochlear nucleus. Identify the striae medullares and the beginning of the trapezoid
body, also the medial and lateral vestibular nuclei (Fig. 107; pp. 149-152). '
60. Section Through the Facial Colliculus. Differentiate between the ventral and
the dorsal portions of the pons, and in the ventral portion identify the longitudinal
fasciculi, transverse fibers, and the nuclei pontis (pp. 147-149). In the dorsal part
identify the nuclei and root fibers of the sixth and seventh nerves including the genu
A LABORATORY OUTLINE OF NEURO-ANATOMY 365
of the seventh nerve. Locate the spinal tract of the fifth nerve and its nucleus, the
trapezoid body, and superior olivary nucleus (Fig. 108; pp. 151-154).
61. Section Through the Middle of the Pans Showing the Motor and Main Sensory
Nuclei of the Fifth Nerve. In addition to these nuclei note the beginning of the mesen-
cephalic root of the fifth nerve. The brachium conjunctivum makes its appearance
in the dorsal part of the section (Fig. 110; pp. 154-157).
62. Section Through the Inferior Colliculus. Identify the basis pedunculi, substantia
nigra, medial and lateral lemnisci, cerebral aqueduct, central gray matter, mesence-
phalic root of the fifth nerve, fasciculus longitudinalis medialis, nucleus of the trochlear
nerve, and the decussation of the brachium conjunctivum (Figs. 113, 114; pp. 158, 165).
63. Section Through the Superior Colliculus. Identify, in addition to the structures
continued upward from lower levels, the red nucleus, the nucleus of the third nerve,
and the root fibers of that nerve, the ventral and dorsal tegmental decussations, the
inferior quadrigeminal brachium, and the medial geniculate body (Fig. 116; pp. 160,
167).
THE CEREBELLUM
64. Compare the human cerebellum with that of the shark and the sheep. How
is its size related to the size of the pons and to the extent of the cerebral cortex?
65. On both the human and sheep's cerebellum identify the vermis, hemispheres,
and divided peduncles (Figs. 138, 139, 143-145). In the medial sagittal section of
the sheep's brain identify the white medullary body of the cerebellum, the arbor
vitae, cerebellar cortex, folia, and sulci (Fig. 84; pp. 196-199).
66. Study the morphology of the cerebellum in the sheep (Figs. 143-145). Lo-
cate these same fundamental subdivisions in the human cerebellum (Figs. 146, 147).
What functions have recently been assigned to each of these subdivisions? (See
pp. 199-203.)
67. Divide the human cerebellum in the median plane. Cut the right half into
horizontal sections and the left into sagittal sections and study the medullary center
and nuclei of the cerebellum (Figs. 140, 141, 148; pp. 199, 203).
68. Study the histologic sections of the cerebellar cortex and master the details
of its structure (Figs. 150, 151; pp. 206-210).
FUNCTIONAL ANALYSIS OF THE BRAIN STEM
69. Review the sections of the brain stem as directed in the following paragraphs,
paying special attention to the functional significance of the various nuclei and fiber
tracts as far as they can be followed in the series of sections. In general, the afferent
tracts and nuclei should be entered in color on the right side of the drawings already
made, and the efferent tracts and nuclei on the left side. But this order must be re-
versed in certain cases to allow for the decussation of the tracts. Label the various
tracts and nuclei. Use the following color scheme:
Somatic afferent:
Exteroceptive blue .
Proprioceptive yellow.
Visceral afferent orange.
Visceral efferent purple.
366 THE NERVOUS SYSTEM
Somatic efferent red.
All cerebellar connections not strictly proprioceptive brown.
Other tracts black.
PROPRIOCEPTIVE PATHS AND CENTERS (pp. 311-315)
70. The cerebellum is the chief proprioceptive correlation center, and the restiform
body consists for the most part of proprioceptive afferent paths (Fig. 235). Note its
shape, position, and connections in all the gross specimens. In the left lateral half of
the sheep's brain follow it caudally by dissection, separating it from the other peduncles.
Cut and reflect the dorsal cochlear nucleus of the eighth nerve. Trace the restiform
body backward and note the accession of external arcuate fibers. At the level of the
inferior olive it receives the dorsal spinocerebellar tract. Trace this by dissection from
the restiform body obliquely across the upper end of the tuberculum cinereum and
then caudally along the ventral border of this elevation to the spinal cord. (See Figs.
87, 88, 104; pp. 143, 205.)
71. Now take the sections of the medulla, locate the dorsal spinocerebellar tract
in each, and indicate its position in yellow on the right side of your outlines (p. 144).
Locate the external ar cute fibers (p. 139). From where do they come and where do they
go? Draw in yellow those belonging to the right peduncle. Locate in your sections
the oliwcerebellar tract, and with brown indicate in your outline the fibers running into
the right peduncle (Fig. 103).
72. From your texts ascertain the course of the ventral spinocerebellar tract and
indicate its position in yellow on the right side of the outlines (Fig. 149; p. 157).
73. Proprioceptive Path to the Cerebral Cortex. Indicate in yellow the terminal
portion of the right dorsal funiculi, and with yellow stipple the right nucleus gracilis
and nucleus cuneatus (Figs. 98, 99). Study the internal arcuate fibers and the medial
lemniscus, drawing the internal arcuate fibers from right to left and the medial lemniscus
on the left side (yellow). Where do the fibers of the medial lemniscus terminate?
What is the source and what the destination of the impulses which they carry? (See
Figs. 101, 103, 107, 108, 110, 114, 116, 235 and pp. 138, 312.)
74. Locate the vestibular nuclei and indicate them with yellow stipple on the right
side of the outlines (Figs. 101, 103, 107, 108). Locate the vestibulocerebellar tract
(pp. 151, 188; Fig. 136).
EXTEROCEPTIVE PATHS AND CENTERS (pp. 302-310)
75. The Cochlear Nerve and its Connections. On the sheep's brain note the two
divisions of the acoustic nerve as well as the ventral and dorsal cochlear nuclei and the
trapezoid body (Fig. 87). Examine the cochlear nuclei and the striae medullares in the
human brain (Fig. 89). Locate the lateral lemniscus where it forms a flat band of
fibers directed rostrally and dorsally upon the lateral surface of the mesencephalon.
It occupies a triangular space dorsal to the basis pedunculi and rostral to the pons and
is superficial to the brachium conjunctivum (Fig. 88).
76. Now take the section through the lower border of the pons and study the
cochlear nuclei, the stria medullares, and the beginning of the trapezoid body (Fig. 107).
In the section through the facial colliculus study the trapezoid body and the superior
A LABORATORY OUTLINE OF NEURO-ANATOMY 367
olivary nuclei (Fig. 108). In the section through the middle of the pons identify the
lateral lemniscus. Trace this tract to the inferior colliculu,s (Fig. 114) and through the
inferior quadrigeminal brachium to the medial geniculate body (Figs. 114, 116). Color
these central connections of the cochlear nerve blue, indicating the cochlear nuclei on
the right side and the lateral lemniscus on the left (Fig. 134; pp. 149, 185).
77. Dissection of the spinal tract of the fifth nerve. On the left half of the sheep's
brain locate the fifth nerve and tear away the transverse fibers of the pons caudal to
that nerve until the longitudinal fibers of its spinal tract are exposed. By carefully
scraping away the structures superficial to this tract follow it to the lower end of the
medulla.
78. Locate the sensory nuclei of the fifth nerve in your sections and indicate them with
colored stipple on the right side of your drawing (pp. 154, 182; Fig. 131): the mesen-
cephalic nucleus, yellow (Fig. 114); the main sensory nucleus, blue (Fig. 110); the
nucleus of the spinal tract, blue (Figs. 98, 99, 101, 103, 107, 108). At the same time
color the spinal tract of the right side blue. What becomes of the fibers which arise
from the cells of the main sensory and the spinal nuclei of the trigeminal nerve? (See
pp. 183, 307; Fig. 232.)
79. From the text ascertain the course of the spinothalamic tract and trace it up
through the brain stem (Figs. 105, 230, 231, 234). Where do these fibers come from,
and where do they end? What kind of sensations do they mediate? Enter it in blue
on the right side of your drawings. (See pp. 101, 102, 145, 305.)
VISCERAL AFFERENT PATHS AND CENTERS
80. Identify the tractus solitarius and its nucleus (Figs. 101, 103, 120). What is
the origin, termination, and function of the fibers constituting this tract? (See pp.
180, 181.) Indicate the tract with orange and the nucleus with orange stipple on the
right side of your drawing.
VISCERAL MOTOR CENTERS
81. In the sections of the brain stem identify the dorsal motor nucleus of the vagus
(Figs. 101, 103) and the following special visceral motor nuclei: the nucleus ambiguus
(Figs. 101, 103), the motor nucleus of the fifth (Fig. 110), and the motor nucleus of the
seventh nerve (Fig. 108). Stipple these nuclei purple on the left side. How are visceral
afferent and efferent elements connected to form visceral reflex arcs? (See pp. 174-178.)
SOMATIC MOTOR TRACTS AND CENTERS
82. The Corticospinal and Corticopontine Tracts. From the cerebral cortex the
fibers of the pyramidal tract run through the internal capsule and brain stem to the
somatic motor and special visceral motor nuclei of the cranial nerves and to the anterior
gray column of the spinal cord. Along with these it will be convenient to study the
cortico-ponto-cerebellar pathway. Take the left lateral half of the sheep's brain and,
being careful not to injure the optic tract and optic radiation, follow the fibers of the
basis pedunculi by dissection through the internal capsule to the cerebral cortex (Fig.
260) . Now tear away the transverse fibers of the pons a few at a time and follow them
by dissection into the brachium pontis. Observe that some of the fibers of the basis
pedunculi end in the pons (corticopontine fibers) and that others (corticospinal fibers)
368 THE NERVOUS SYSTEM
can be traced through the pons into the pyramid of the medulla. Carrying the dis-
section caudally, observe the decussation in the lower end of the medulla.
83. Examine again the series of sections through the brain stem and color the
corticospinal tract red on the right side of your drawings. Draw the fibers from right
to left in the decussation (Fig. 237; pp. 136, 317).
84. With red stipple indicate the somatic motor nuclei on the left side of your draw-
ings. Which nuclei are they? (See pp. 170-173.)
CEREBELLAR CONNECTIONS
85. The inferior peduncle has already been studied and the cortico-ponto-cerebellar
path has been dissected. Review this path in your sections. Color the corticopontine
tracts of the left side brown ( Fig. 117). Indicate the nuclei pontis of the left side by
brown stipple. Draw the transverse fibers of the pons from the left nuclei pontis to
the right brachium pontis (Fig. 106; pp. 147-149).
86. In the left lateral half of the sheep's brain follow the brachium conjunctivum
by dissection into the tegmentum of the mesencephalon and note its decussation
beneath the inferior colliculus. In your sections trace it rostrally, noting its decus-
sation and termination (Figs. 110, 112, 114-116). Indicate it in brown on your
drawings, beginning on the right side and tracing it through the decussation to the left
red nucleus. Stipple both red nuclei with brown. (See pp. 159, 326.)
87. The Rubrospinal Tract. Trace the rubrospinal tract from the red nucleus
through the ventral tegmental decussation (Fig. 116) and the reticular formation of the
brain stem. In the reticular formation it occupies a position ventromedial to the
nucleus of the spinal root of the trigeminal nerve (Figs. 115, 234; pp. 161, 326). Color
it brown on the left side of your drawings.
THE RETICULAR FORMATION
88. Study the reticular formation in the various sections. Of what is it composed?
How many kinds of internal arcuate fibers can you find? What is the source of the
longitudinal fibers of the reticular formation? Locate the tectospinal tract and in-
dicate it in black on the left side of your drawings. (See pp. 144, 145).
89. The Fasciculus Longitudinalis Medialis. Examine all nine sections, and enter
this bundle in black on both sides of your drawings. What is the source of its fibers
and what is its function? (See Fig. 109; pp. 152, 162).
PROSENCEPHALON
90. With a sharp brain knife divide the human brain exactly in the median sagittal
plane, and then cut the left cerebral hemisphere into a series of frontal sections. The
planes of the sections should pass through (1) the rostrum of the corpus callosum,
(2) the anterior commissure, (3) the mammillary body, (4) the habenular nucleus,
(5) the pineal body and the splenium of the corpus callosum (Figs. 186-190).
91. Take the right half of the sheep's brain and make such dissections as may be
necessary to secure a good preparation of the structures indicated in Fig. 84. Begin
at the rostral angle of the fourth ventricle and follow the cerebral aqueduct, tearing
away with tissue forceps any parts of the left lateral wall which have not been cut away.
A LABORATORY OUTLINE OF NEURO-ANATOMY 369
Follow the aqueduct into the third ventricle, removing from the latter the remains of
its left lateral wall. Care is required in removing the rostral part of this wall in order
that the lamina terminalis may be left intact. Now remove such portions of the left
cerebral cortex as are still attached to the preparation. By this dissection a much more
instructive preparation is obtained than when the original section is made exactly in
the median plane.
92. Take the left lateral hah of the sheep's brain and tear away what remains of the
septum pellucidum and body of the fornix and locate the caudate nucleus. For the
identification of these structures see Figs. 84 and 204. ^ Cut through the internal capsule,
which has previously been exposed from the lateral side in this specimen, along a line
extending horizontally toward the occipital pole from the highest part of the dorsal
border of the caudate nucleus. Remove the portion of the cerebral hemisphere that
lies dorsal to the plane of this section and thus expose the dorsal surface of the thalamus
(Fig. 91).
93. Diencephalon. Study the thalamus as it appears in all of these preparations
(pp. 213-216). Examine the dorsal surface of the thalamus on the left half of the sheep's
brain (Figs. 89, 91, 180). The lateral surface of the thalamus rests against the internal
capsule, as can be readily understood from a study of this dissection. The medial
surface forms a part of the wall of the third ventricle (Figs. 158, 159).
94. Study the epithalamus in both the human and the sheep's brain. Of what
parts is it composed? (See Figs. 91, 158, 159; pp. 220, 221.)
95. Locate all the parts which belong to the hypothalamus in both the human and
the sheep's brain (Figs. 84, 86, 158, 159; pp. 222, 223).
96. Study the shape and boundaries of the third -ventricle (Figs. 158, 159; pp.
223, 224).
97. The Metathalamus. On the left half of the sheep's brain identify the medial
geniculate body (Fig. 87). Immediately rostral to this body is a slight elevation in the
optic tract produced by the subjacent lateral geniculate body. Identify both of these
bodies on the human brain (Figs. 88, 89, 154).
98. In the frontal sections of the left human cerebral hemisphere identify the various
parts of the diencephalon (Figs. 188, 189). From these sections something can be
learned concerning the internal structure of the thalamus, but more information can
be obtained on this subject from sections stained by the Weigert method (Figs. 156,
157; p. 216). In these sections trace the basis pedunculi into the internal capsule and
the medial lemniscus into the thalamus.
99. Dissection of the Optic Tract Take the left lateral half of the sheep's brain
and, grasping the optic chiasma with the tissue forceps, pull the optic tract lateralward,
separating it from the surface of the peduncle. It separates easily until the position
of the lateral geniculate body is reached just rostral to the medial geniculate body.
Stronger traction will cause it to tear away from the lateral geniculate body, which is
now exposed as a prominent curved ridge of gray matter. This nucleus extends rostrally
and dorsally from the medial geniculate body and is continuous with the pulvinar of
the thalamus. Continued traction will cause the optic fibers to strip off from the sur-
face of the pulvinar. Here they form a rather thick white lamina, the stratum zonale.
Continue the dissection, raising the fibers of the optic tract as far as the groove rostral to
the superior colliculus. Now cut the transverse peduncular tract, which lies in this
24
THE NERVOUS SYSTEM
groove, by making a superficial incision across the groove along the lateral border of
the optic fibers. Scrape away the superficial gray matter (about 1 mm.) of the superior
colliculus and expose the stratum opticum (Fig. 116). Now continue the traction on
the optic tract and a striking demonstration will be obtained of the fact that the stratum
opticum is composed of fibers from this tract (Figs. 161, 162; pp. 226, 227).
100. Dissection of the Optic Radiation. In the left half of the sheep's brain scrape
away part of the gray matter of the pulvinar. Follow fibers from the pulvinar into the
posterior limb of the internal capsule. These belong to the optic radiation, which may
now be followed by dissection to the cortex near the occipital pole of the cerebral hemi-
sphere (Fig. 260; pp. 227, 228). Now take the right half of the cerebral hemisphere
and identify the visual area of the cerebral cortex (Fig. 221).
Optic radiation ' ' /*
Superior colliculus:' /
Inferior colliculus ' \
Pulvinar '^|
Medial geniculate body '"
Cerebral peduncle
Mammillary body
Optic tract
Posterior limb of internal capsule
Optic nerve
Intersection of corona radiata and
radiation of corpus callosum
Anterior limb of internal capsule
Anterior perforated substance
Fig. 260. Dissection of the cerebrum of a sheep showing the internal capsule and corona radiata.
The lentiform nucleus has been removed.
101. Surface Form of the Cerebral Hemispheres. Compare the basal surface of the
human brain with that of the sheep. Note in each the parts belonging to the rhinen-
cephalon and locate the rhinal fissure, which separates the neopallium and the archi-
pallium. Nearly all of the surface of the human cerebral hemisphere is formed by the
neopallium (Figs. 83, 86; pp. 115, 116).
102. Examine the right cerebral hemisphere of the human brain and identify the
poles, fissures, sulci, lobes, and gyri (Figs. 166-168, 170, 171; pp. 232-242). Draw
the margins of the lateral fissure apart and locate the insula (Fig. 169). Study
the insula in the frontal sections through the left cerebral hemisphere (Figs. 186-189;
p. 237).
103. Internal Configuration of the Cerebral Hemisphere. Take the sheep's brain
from which the cerebellum has been removed and slice away successive thin layers from
the dorsal aspect of both hemispheres. These thin sections should be cut in planes
parallel to the dorsal surface of the corpus callosum and the last cut should be inch
dorsal to that commissure. The direction and relative depth of the dorsal surface of
A LABORATORY OUTLINE OF NEURO-ANATOMY 371
the corpus callosum can be determied by examination of the medial aspect of the right
half of the sheep's brain. As the sections are removed note the relation of the gray
and white matter (Fig. 175). Gently press apart the two hemispheres and note the corpus
callosum at the bottom of the longitudinal fissure. Now with a blunt instrument
dissect away the gray and white matter from the dorsal surface of the corpus callosum
(Fig. 175). Be careful not to injure a thin layer of gray matter, the indusium griseum,
which covers this surface. Study the corpus callosum in this specimen and in the median
sagittal sections of the sheep and human brains (Figs. 158, 159, 175; pp. 243-245).
Examine the septum pellucidum in the median sagittal sections.
104. The Lateral Ventricles (pp. 246-251). Cut through the corpus callosum of the
sheep's brain as indicated in Fig. 178, leaving a median strip in position. Make a
careful examination of all the parts thus exposed, including the septum pellucidum.
On the right side of the specimen expose the entire extent of the inferior horn of the
lateral ventricle by freely cutting away the lateral portion of the hemisphere as indicated
in Fig. 182. Remove the caudate nucleus to demonstrate the entire extent of the ante-
rior horn, and finally demonstrate the continuity of the lateral ventricle with the cavity
of the olfactory bulb (Fig. 182). Now study the lateral ventricle and the structures
which form its walls as these are illustrated on the two sides of this specimen. Note
the chorioid plexus (Fig. 183) and chorioid fissure.
105. Study the lateral ventricle as seen in the frontal sections of the left hemi-
sphere of the human brain (Figs. 186-189). It has an additional part, the posterior
horn, not seen in the sheep. Endeavor to reconstruct a mental picture of its shape
(Fig. 176).
106. The Corpus Striatum (pp. 253-257). Examine again the caudate nucleus as
it bulges into the lateral ventricle (Fig. 178). Take the right lateral half of the sheep's
brain and make a horizontal section through the cerebral hemisphere, passing through
the lower border of the genu of the corpus callosum and the lower border of the habenular
trigone. Locate the lentiform and caudate nuclei, the claustrum, and the internal
and external capsules (Fig. 192).
107. Dissection of the Lentiform Nucleus and the Internal Capsule. On the left
side of the sheep's brain, in which the lateral ventricles have been exposed, remove the
cortex and white matter superficial to the lentiform nucleus. Begin by grasping with
tissue forceps the olfactory bulb close to its peduncle and tear it away, pulling in a
lateral and caudal direction. There should come away with it the superficial part of
the anterior perforated substance and part of the lateral olfactory gyrus (Fig. 83).
This will expose the ventral part of the lentiform nucleus, and the structures lateral
to that nucleus can now be removed. With a blunt dissecting instrument scrape away
everything superficial to the lentiform nucleus and continue the dissection until the
nucleus and the corona radiata are fully exposed (Fig. 87). Now scrape away the
lentiform nucleus and expose the internal capsule (Fig. 260). In removing the nucleus
you can obtain a clear idea of its shape and size.
108. Dissection of the Internal Capsule. In the same specimen remove the optic
tract and trace the basis pedunculi into the internal capsule and follow the fibers from
the internal capsule into the corona radiata. Trace the optic radiation from the poste-
rior extremity of the internal capsule to the cortex near the occipital pole (Fig. 260).
109. Dissection of the Caudate Nucleus. On the left side of the same sheep's
372 THE NERVOUS SYSTEM
brain note that the tail of the caudate nucleus extends ventrally into the roof of the
inferior horn of the lateral ventricle. With a blunt instrument scrape away the head
and first part of the tail of the nucleus, exposing the medial surface of the internal cap-
sule (Fig. 91). Note the shape and size of this nucleus as you are removing it.
110. Study a horizontal section stained by the Weigert method through the internal
capsule and basal ganglia. From this section and from the dissections endeavor to
form a clear mental picture of the internal capsule and its relations (Figs. 191, 193;
pp. 257-261).
111. Now take the frontal sections of the left hemisphere of the human brain
and identify the various parts of the corpus striatum and internal capsule (Figs. 186-
190).
112. Rhinencephalon. Study the olfactory portions of the brain to be seen on the
ventral surface of the cerebral hemisphere in the human and sheep's brains (Figs. 172,
197, 199 ; pp. 265-269) . Study the hippocampus, alveus, and fimbria as they lie exposed
in the inferior horn of the lateral ventricle of the sheep's brain (Figs. 178, 182). Open
up the inferior horn of the lateral ventricle on the left side of this specimen so as to
expose the hippocampus and fimbria. Raise the hippocampus and fimbria on both sides
at the same time, leaving them still attached to the fornix. This should be done without
damaging the underlying tela chorioidea of the third ventricle, which occupies the great
transverse fissure. Examine the under surface of the hippocampus, fimbria, and for-
nix. Note that the two fimbriae unite to form the triangular body of the fornix.
The transverse fibers in this triangle constitute the hippocampal commissure (lyra).
Note the fascia dentata and hippocampal fissure. Figure 204 will help you to interpret
the parts seen in this dissection.
113. The chorioid plexuses of the prosencephalon are now fully exposed, and their
relations to each other and the brain ventricles can be readily studied (pp. 224, 251).
114. Remove the tela chorioidea of the third ventricle and again identify the parts
of the thalamus and epithalamus which may be seen from above (Figs. 91, 180).
115. Replace the fornix and hippocampus in position and divide the fornix and what
remains of the cerebral hemispheres by a sagittal section \ millimeter to the right of
the median plane. Take the left half of the preparation and, tearing away any por-
tions of the right columna fornicis that may still be attached to the preparation, follow
the left column of the fornix to the mammillary body. This can be accomplished by
scraping away some of the medial surface of the thalamus (Fig. 204). At the same time
expose the mamillothalamic tract. Remove the posterior part of the thalamus and the
remainder of the brain stem by a cut made just caudal to the mamillothalamic tract,
as indicated in Fig. 204. This gives a connected view of the entire fornix system.
Find the cut surface of the hippocampal commissure and separate it for a few milli-
meters from the rest of the fornix. Identify again the fimbria, fascia dentata, hippo-
campal fissure and hippocampal gyrus, and study the fornix as a whole (Figs. 200,
203; pp. 270-272).
116. Study the septum pellucidum in the right half of the human brain (Fig. 158;
p. 272). Also locate the anterior commissure.
117. Dissect the anterior commissure in the right lateral half of the sheep's brain.
Locate the commissure on the median surface and by blunt dissection follow it to the
olfactory bulb (Fig. 199; p. 273).
A LABORATORY OUTLINE OF NEURO-ANATOMY 373
118. In the frontal sections of the left cerebral hemisphere of the human brain study
the relations of the septum pellucidum, fornix, fimbria, hippocampus, and anterior
commissure (Figs. 186-190).
119. The Cerebral Cortex. On the right hemisphere of the human brain identify
the motor, somesthetic, auditory, and visual centers (Figs. 220, 221; pp. 290-293).
With a scalpel remove a cube of cortex and subjacent white matter from each of these
areas. Each block should measure about 1 cm. in each dimension. With a sharp
razor make section through each of these blocks at right angles to the surface of the cortex
and perpendicular to the long axis of the gyrus from which the block was cut. Note the
differences in thickness of the cortex in the various regions. Observe the white
striations in the cortex, and note how these differ in the several specimens (Fig. 218).
Study the stained and mounted sections of the cerebral cortex which are furnished
you. What details of cell and fiber lamination do these preparations show, and how
does this lamination differ in the several regions of the cortex? (See Fig. 215; pp.
284-287.)
120. Association Fibers (Figs. 226, 228; pp. 298-301). If the human brain is reason-
ably well preserved the larger bundles of association fibers may be easily exposed by
dissection. This can be done on the right hemisphere. But if the material is very
soft this half of the brain can more profitably be laid into a series of horizontal sections
and these used for a review of the form and relations of the component parts of the
cerebral hemisphere. If the material is fairly well preserved, make the following
review dissection and at the same time expose and study the various bundles of asso-
ciation fibers.
121. Review Dissection of the Human Brain. Take the right half of the
human brain and scrape away the cerebral cortex from a portion of the dorsal
surface of the frontal lobe. This will expose the short association or arcuate fibers
(Fig. 226).
122. Now make a horizontal section through the hemisphere parallel to the dorsal
surface of the corpus callosum and J- inch dorsal to it. Note the centrum semiovale.
Scrape away the cortex of the gyrus cinguli and the white matter immediately sub-
jacent to it. In making this dissection carry the orangewood stick in an anteroposterior
direction, removing the white matter a little at a time until a longitudinal bundle of
fibers, the cingulum, is exposed (Fig. 174). The indusium griseum and striae longi-
tudinales should now be uncovered.
123. Remove the cingulum, scrape away the indusium griseum, and expose the
radiation of the corpus callosum as indicated on the right side of Fig. 174, but do not
cut the optic radiation or expose the tapetum at this time.
124. Remove the parietal operculum a little at a time. This can be done with
tissue forceps. Grasp small portions and t^ar them away by upward traction. Note
the bundles of transverse fibers which enter this operculum from the corpus callosum
and internal capsule. These intersect at right angles with the fibers of the superior
longitudinal fasciculus which should come into view as the dissection progresses (Fig.
174). The transverse bundles should be made to break off at the point where they pass
through the superior longitudinal fasciculus. Complete the dissection of this fasciculus,
carrying the dissecting instrument in the direction of its fibers. Now demonstrate the
intersection of the corona radiata w r ith the radiation of the corpus callosum (Fig. 174).
374
THE NERVOUS SYSTEM
By this dissection the insula and the dorsal surface of the temporal lobe have been ex-
posed. Note in particular the transverse temporal gyri.
125. Now dissect away the dorsal part of the temporal lobe and remove the insula.
This will expose the uncinate and inferior occipitofrontal fasciculi as well as the external
capsule (Fig. 227). These fiber bundles can best be displayed by carrying the dis-
secting instrument in the direction of the fibers. Complete the dissection of the corona
radiata and the optic radiation (Fig. 227).
126. Now turn the specimen over and make a dissection of the column ofthefornix
and the mamillothalamic tract as in Fig. 205, but do not cut away the brain stem as
indicated in that figure.
127. Dissection of the Internal Capsule from the Medial Side (Fig. 195). Tear
away the fornix and septum pellucidum, opening up the lateral ventricle. With the
brain knife cut away a slice from the medial surface of the hemisphere, varying in thick-
ness from T inch at the frontal end to i inch at the occipital end, cutting through the
corpus callosum and into the ventricle, but not into the basal ganglia. With a scalpel
and tissue forceps remove what remains of the medial wall of the lateral ventricle,
except in the inferior horn. Grasp with tissue forceps the stria terminalis in the rostral
end of the sulcus terminalis and tear it away, carrying the forceps toward the occipital
pole (p. 214). By blunt dissection remove the thalamus and subthalamus as well as the
tegmentum and corpora quadrigemina of the mesencephalon. In scraping away these
parts carry the dissecting instrument from the sulcus terminalis in a ventral direction.
This will uncover the basis pedunculi and its continuation into the internal capsule.
The fibers of the thalamic radiation will be broken off at the point where they enter the
internal capsule (Fig. 195). Remove the ependymal lining of the posterior horn of the
ventricle and uncover the tapetum. Scrape away the caudate nucleus, carrying the
dissecting instrument in the direction of the fibers of the internal capsule (Fig. 195).
Trace the anterior commissure to the point where it disappears under the anterior
limb of the internal capsule. Study the internal capsule as seen from the medial sur-
face, and note particularly the direction of the fibers, the anterior limb, the posterior
limb, the optic radiation, and the curved ridge which represents the genu,
128. Now turn again to the lateral side of the specimen (Fig. 227), and grasping
with tissue forceps individual strands of the uncinate fasciculus in temporal lobe strip
them forward into the frontal lobe. Remove the entire fasciculus in this manner. In
the same way strip away the fibers of the inferior occipitofrontal fasciculus, beginning
in the frontal lobe and tracing them toward the occiput. Strip off the fibers of the ex-
ternal capsule and expose the lentiform nucleus and the corona radiata (Fig. 194).
Pay special attention to the fibers of the corona radiata which come from the sublen-
ticular part of the internal capsule and enter the temporal lobe. Follow the anterior
commissure to the point where it disappears under the lentiform nucleus.
129. Remove what remains of the temporal lobe and examine the hippocampus,
fimbria, and inferior horn of the lateral ventricle from the dorsal surface (Fig. 201).
130. Next scrape away the lentiform nucleus and trace the basis pedunculi into the
internal capsule (Fig. 88). Study the corona radiata, internal capsule, and basis
pedunculi from both sides of this preparation. The thalamus and the caudate and
lentiform nuclei produce well-marked impressions on the internal capsule (Figs. 88, 195).
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INDEX
NOTE. In cross references the key words are italicized.
the pages on which the structures are illustrated.
The numbers in Italics refer to
ACCOMMODATION of vision, 332
Acoustic area of cortex. See Center, auditory.
Acousticolateral area, 358
Affenspalte, 237
Ala cinerea, 127
lobuli centralis, 797
Alveus, 270, 278, 279
Ameba, 17
Ammon's horn. See Hippocampus.
Ampulla of semicircular canal, 358
Amygdala. See Nucleus, amygdaloid.
Ansa lenticularis, 263
peduncularis, 263
Aperture, lateral, of fourth ventricle, 125
medial, of fourth ventricle, 125
Apex columnse posterioris, 79
Aphasia, 295
Aqueductus cerebri (aqueduct of Sylvius), 26,
158
Arachnoid, 73
Arbor vitae, 199
Archipallium, 116, 242, 270, 277, 278, 279
Area, acousticolateral, 358
acustica, 127
cortical, 287. (See also Center.)
oval, of Flechsig, 107
parolfactoria of Broca, 267
postrema, 129
pyriform, 116, 268, 277
striata, 293
Association bundles of cerebrum, 298
arcuate, 298, 300
cingulum, 299
inferior longitudinal, 299
occipitofrontal, 300
superior longitudinal, 300
occipitofrontal, 301
uncinate, 299
Ataxia, 99
Auditory apparatus, 186, 309
Auerbach's plexus, 351
Autonomic system, 339
cranial, 339
craniosacral, 340, 354
sacral, 339
thoracicolumbar, 339, 354
Axon (axis-cylinder), 37, 43, 45
hillock. See Cone, implantation.
Axonal reaction. See Chromatolysis.
BAILLARGER, lines of, 283
Band, diagonal, 267
Basis cerebri, 115, 120
pedunculi, 129, 158, 164
Basket-cells, 209
Bell's law, 60
Betz, cells of, 290
Bladder, innervation of, 354
Body of cell, 43
of fornix, 271
geniculate, lateral, 131, 220
medial, 131, 167, 220
mammillary, 222, 280
of Nissl, 48, 51
paraterminal, 267
pineal, 221
pituitary. See Hypophysis.
quadrigeminal, 130, 165
restiform, 122, 143, 205
striate. See Corpus striatum.
tigroid. See Nissl body.
trapezoid, 121, 150, 186
Brachium (or brachia), conjunctivum, 125, 155,
159, 160, 206, 211
of corpora quadrigemina, 131
pontis, 123, 204
quadrigeminum inferius, 131, 163, 166
superius, 131, 167
Brain, 56, 113
development, 25
divisions of, 25
end-. See Telencephalon.
fore-. See Prosencephalon.
hind-. See Metencephalon and Rhombenceph-
alon.
inter-. See Diencephalon.
stem. See Medulla oblongata, Pans, Mesen-
cephalon, and Ganglia, basal.
vesicles, 24, 25
weight, 301
Broca's convolution, 235
Brown-Sequard syndrome, 112
Bulb, olfactory, 265, 274
of posterior horn, 248
Bundle. (See also Fasciculus and Tract.}
association, of cerebrum, 298, 299, 300
cornucommissural, 107
ground. See Fasciculus proprius.
of Gudden, tegmental. See Tract, mammillo-
tegmental.
marginal. See Fasciculus dorsolateralis.
oval. See Area, oval.
posterior longitudinal. See Fasciculus, medial
longitudinal.
of Turck. See Tract, ventral corticospinal.
ventral longitudinal. See Tract, tectospinal.
Burdach, column of. See Fasciculus cuneatus.
nucleus of. See Nucleus cuneatus.
CAJAL, commissural nucleus of, 330
horizontal cells of, 285
Calamus scriptorius, 127
383
384
INDEX
Calcar avis, 238, 248
Canal, central (canalis centralis), 80, 136
lateral line, 356
semicircular, 315, 356
spinal, 73
Capsule, external, 257
internal, 257, 259, 261
nasal, 356
of spinal ganglion cell, 63
Cauda equina, 78
Cavum septi pellucidi, 272
Cell. (See also Neuron.)
basket, 209
of Betz, 290
body, 43
ependymal, 37, 85
germinal, 37
granule, of cerebellum, 208
of cerebral cortex. See Neurons, stellate.
of olfactory bulb, 276
mitral, 275
neuroglia, 85, 86
of Purkinje, 207
pyramidal, 285
Cell-columns of Clarke. See Nucleus dorsalis.
intermediolateral, 89
of spinal cord, 89, po
Center, cortical, 290
association, 293
auditory, 293
motor, 290, 317, 318
olfactory, 293
optic, 292
projection, 290
somesthetic, 292
of speech, 295
visual, 292
for pain, 219
projection, 290
respiratory, 330
Central nervous system, 20, 21, 56, 57
Centrum medianum thalami, 218
semiovale, 243
Cerebellum, 195
in birds and reptiles, 200
central white matter, 199
cortex, 199, 206, 207, 208, 209
development of, 195
in the dogfish, 27, 28
fiber tracts of, 204, 205, 206, 209, 210, 211
folia, 199
hemispheres of, 197, 198
histpgenesis, 196
laminae, 199
lobes or lobules, 197, 198, 200, 201, 202
in mammals, 200
microscopic structure, 206
morphology of, 199
notches, 197
nucleus dentatus, 203, 211
emboliformis, 203
fastigii or tecti, 204, 211
globosus, 203
peduncles, 204
inferior, 122, 143, 205
middle, 123, 204
superior, 125, 155, 159, 160, 206, 211
section, median, 199
through hemisphere, 199
Cerebellum in the sheep, 200, 201, 202
vermis of, 196
white matter, 199
Cerebral aqueduct. See Aqueductus cerebri.
cortex, 114, 232, 283
area of, acoustic, 293
association, 293
audito-psychic, 293, 294
audito-sensory, 294
of Broca, 295
motor, 290, 317, 318
striata, 293
visuo-psychic, 293, 294
visuo-sensory, 294
centers of, 290, 292, 295
. development, 230
electric excitability of, 291
frontal olfactory, 277
hippocampal, 278, 279
histogenesis, 230
layers of, 286, 287
localization of function in, 290
myelination of fibers, 289
nerve-cells, 284, 285
nerve-fibers, 283, 284
neuroglia-cells, 284
structure, 283, 284, 285, 286
hemispheres, 113, 229, 232
borders, 232
commissural fibers, 296
convolutions. See Gyri.
corticifugal or efferent fibers, 283
corticipetal or afferent fibers, 283
development, 25, 32, 229
in the dogfish, 27, 28, 30
external conformation, 229
fissures. See Fissure.
gyri. See Cyrus.
lobes. See Lobe.
lobules. See Lobule.
medullary center, 243, 296
pallium, 25, 32, 33, 229
poles, 232
sulci. See Sulcus.
surfaces, 232
ventricles, lateral, 246 .'.
peduncles. See Peduncles.
vesicles, 24, 25
Cerebrospinal fluid, 73, 126
system, 58
Cerebrum, 117
Cervix, columnae posterioris, 79
Chiasma, optic, 223, 226
Chorda tympani, 192, 352
Chorioid fissure, 229, 251
plexuses. See Plexus.
Chromatolysis, 51
Chromophilic bodies. See Nissl bodies.
Cingulum, 299
Clarke, column of. See Nucleus dorsalis.
Claustrum, 256
Clava, 121, 137
Climbing fibers, 209, 210
Clivus monticuli. See Declive monticuli.
Cochlea, 185
Coelenterates, 19
Cold, sensations of, 105, 306
Collateral fibers, 43, 97
Colliculus facialis, 127
INDEX
385
Colliculus, inferior, 130, 165
superior, 130, 165, 167
Column, anterior, 80
of Burdach. See Fasciculus cuneatus.
of Clarke. See Nucleus dorsalis.
dorsal (columna dorsalis grisea), 42
of fornix, 272
of Goll. See Fasciculus gracilis.
gray, 79
inter mediolateral, 89
lateral, 80
nuclear, of brain stem, 168, 170, 171, 174
posterior, 79
somatic afferent, 170, 182, 185
efferent, 170
ventral, 42, 80
vesicular. See Nucleus dorsalis.
visceral afferent, 170, 180
efferent, 170, 174, 177
Comma tract of Schultze. See Fasciculus inter-
fascicularis.
Commissura anterior alba, 80
grisea, 80
habenularum, 220
Commissure or commissures, anterior cerebri,
223, 231, 273, 296
gray, 80
white, 80
great transverse. See Corpus callosum.
of Gudden, 227
habenular, 220
hippocampal, 231, 271, 280, 296
of inferior colliculi, 759
middle. See Massa intermedia.
optic. See Chiasma, optic.
posterior, of cerebrum, 221
of spinal cord, 80
superior. See Commissure, habenular.
Components of nerves, 61, 168. (See also
Nerve-fibers.)
Conduction of nerve impulses, 50
Cone, implantation, 44
of origin. See Cone, implantation.
Cones of retina, 226
Consciousness, 23, 302
Conus medullaris, 74
Convolution. See Cyrus.
Coordination, 99, 210, 311
Cornu ammonis. See Hippocampus.
Cornucommissural bundle, 107
Corona radiata, 261
Corpus (or corpora) callosum, 243, 296
development, 231
fornicis, 271
geniculatum laterale, 220
mediale, 131, 167, 220
mamillaria, 222, 230
pineale, 221
ponto-bulbare, 123
quadrigemina, 130, 165
restiforme, 122, 143, 205
striatum, 25, 32, 33, 256, 262, 324
subthalamicum (Luysi), 223
trapezoideum, 121, 150, 186
Cortex, cerebellar, 199, 206, 207, 208, 209
localization of function in, 202
neurons of, 207, 208, 209
cerebral. See Cerebral cortex.
Corti, ganglion of. See Ganglion, spiral.
25
Corti, organ of, 185, 186
Cough, mechanism of, 331
Crus (or crura) cerebri. See Peduncle, cerebral.
fornicis, 271
Crusta. See Basis pedunculi.
Culmen monticuli, 198
Cuneate tubercle, 121, 137
Cuneus, 239, 292
Cup, optic, 32, 33, 225
Cytoplasm of nerve-cells, 42, 47, 48
DECLIVE monticuli, 198
Decussation (decussatio) of brachium conjunc-
tivum, 156, 159
dorsal tegmental, 161, 167
of fillet. See Decussation of lemniscus.
of Forel. See Decussation, ventral tegmental.
fountain. See Decussation, dorsal tegmental.
of lemniscus (lemniscorum), 134, 138
of Meynert. See Decussation, dorsal tegmental.
optic. See Chiasma, optic.
of pyramids, 119, 120, 134, 136
tegmental. See Decussations, ventral and
dorsal tegmental.
ventral tegmental, 161
Degeneration of fiber tracts, 105, 106, 107
of nerve-fibers, 51, 52
Wallerian, 105, 106, 107
Deiters, nucleus of, 151, 189
Dendrites or dendrons, 43
Dermatome, 58
Development of the nervous system, 24, 31
Diencephalon, 24, 25, 26, 28, 31, 33, 213
Digitationes hippocampi, 269
Dogfish, brain of, 26, 27, 28
Dogiel's Type II cells, 65
Dura mater, 73
Dynamic polarity, law of, 50
EARTHWORM, nervous system of, 19
Edinger-Westphal nucleus, 178
Effector, 18, 19, 54, 91
Embryology of nervous system, 31, 37, 195, 213,
229
Eminentia cinerea. See Ala cinerea.
collateralis, 250
facialis. See Colliculus facialis.
hypoglossi. See Trigonum hypoglossi.
medialis, 129
teres. See Eminentia medialis.
Encephalon. See Brain.
End-brain. See Telencephalon.
End-plates, motor, 62
Ependyma, 85
Epiphysis, 29, 31
Epithalamus, 29, 35, 220
Exteroceptor, exteroceptive, 66, 182, 185, 304
Eye, development, 225
innervation, 225
retina, 225
FASCIA dentata, 269, 279
Fasciculus, 95. (See also Tract and Bundle.)
anterior proprius, 107
anterolateralis superficialis, 100
arcuatus, 300
cerebellospinalis. See Tract, dorsal spinocere-
bellar.
cerebrospinalis. See Tract, corticospinal.
3 86
INDEX
Fasciculus cerebrospinalis, anterior. See Tract,
ventral corticospinal.
lateralis. See Tract, lateral corticospinal.
cuneatus, 76, 83, 95, 96, 121, 137
dorsal longitudinal (Schutz), 216
dorsolateralis (Lissauer), 79, 87, 98, 104
gracilis, 76, 83, 96, 121, 137
interfascicularis, 97, 107
lateralis, minor, 121
proprius, 107
longitudinalis inferior, 299
medialis, 145, 152, 162, 190, 328
superior, 300
medial longitudinal, 145, 152, 162, 190, 328
of Meynert, 220
occipitofrontalis, inferior, 300
superior, 301
peduncularis transversus, 369
posterior longitudinal. See Fasciculus, medial
longitudinal.
proprius of spinal cord, 107
pyramidal. See Tract, corticospinal.
retroflexus, 220
septomarginal, 97, 107
solitarius, 132, 181, 330
sulcomarginalis, 108
superior longitudinal, 300
thalamornamillaris. See Tract, mammillo-
thalamic.
uncinatus, 299
Fibers, fibrae. (See also Nerve-fibers.)
arcuate, of cerebrum, 299
of medulla oblongata, 139
external, 121, 123, 139, 140, 143
internal, 134, 138, 139
association, 92, 298
cerebello-olivary. See Fibers, olivocerebellar.
climbing, 209, 210
commissural, 296
mossy, 209, 210
olivocerebellar, 139, 142, 143, 205
pontis, 147
postganglionic, 337, 343
preganglionic, 337, 344
projection, 297
propriae. See Fibers, arcuate, of cerebrum,
rectae, 148
Fila lateralia pontis, 148
Fillet. See Lemniscus.
Filum durae matris spinalis, 74
terminale, 74
externum, 74
internum, 74
Fimbria hippocampi, 250, 269
Final common path, 94, 311
Fissure (or fissura), calcarine, 238, 292
callosal. See Sulcus of corpus callosum.
callosomarginal, 240
central, of Rolando, 233
cerebri lateralis, 233
chorioid, 229, 251
collateral, 239
dentate. See Fissure, hippocampal.
development, 230, 231
great longitudinal, 232
transverse. See Fissure, transverse cere-
bral.
hippocampal, 239, 269, 270
lateral cerebral, 233
Fissure, longitudinal cerebral, 114, 232
mediana, anterior, of medulla oblongata, 119
of spinal cord, 76, 82
posterior, of medulla oblongata, 119
parieto-occipital, 239
prima, 196, 199
rhinal, 116, 240
of Rolando. See Sulcus, central.
secunda, 203
Sylvian. See Fissure, lateral cerebral.
transverse cerebral, 213
Flechsig, direct cerebellar tract of. See Tract,
dorsal spinocerebellar.
Flexure, cephalic, 31, 33
cervical, 32, 33
pontine, 31, 33
Flocculus, 199
Fluid, cerebrospinal, 73, 126
Folium vermis, 198
Foramen caecum, 119
interventricular, 26, 118
of Luschka. See Aperture, lateral, of fourth
ventricle.
of Majendie. See Aperture, medial, of fourth
ventricle.
of Monro. See Foramen, interventricular.
Forceps, major, 245
minor (frontal part of radiation of corpus cal-
losum).
Fore-brain. See Prosencephalon.
Forel, fountain decussation of. See Decussa-
tion, ventral tegmental.
Formatio reticularis, 80, 136, 144
Fornix, 270, 280
body, 271
columns, 271, 272
commissure, 271, 280
crura, 271
fimbria, 270, 271
longus, 282
Fossa interpeduncularis, 115
rhomboid, 126, 127
Fountain decussations of Forel and of Meynert,
161, 167
Fovea, inferior, 127
superior, 127
Frenulum veli medullaris anterior, 130
Frog, sympathetic ganglia of, 344, 345
Funiculus, 95
anterior, 76, 82
cuneatus, 121, 137
dorsal. See Funiculus, posterior.
gracilis, 121, 137
lateralis, 76, 82
posterior, 76, 82
separans, 129
teres. See Eminentia medialis.
ventral. See Funiculus, anterior.
GANGLIATED cord. See Trunk, sympathetic.
Ganglion or ganglia, autonomic. See Ganglia,
sympathetic.
basal, 252
celiac, 349
cerebrospinal (sensory ganglia on the cerebro-
spinal nerves), 38
cervical, inferior, 348
middle, 348
superior, 347
Ganglion, ciliary, 351
of Corti. See Ganglion, spiral.
enteric, small ganglia of myenteric and sub-
mucous plexuses, 351
of facial nerve. See Ganglion, geniculate.
Gasserian. See Ganglion, semilunar.
geniculate, 192
habenula?, 29, 220
interpeduncular, 115, 164
jugular, 193
mesenteric, 349
nodosum, 193
otic, 351
petrosal, 193
of Scarpa. See Ganglion, vestibular.
semilunar, 191
sensory, 38
sphenopalatine, 351
spinal, 62
development of, 38, 40
structure of, 63, 64, 65
spiral, 185, 186
submaxillary, 351
sympathetic, collateral, 335
development of, 41, 335
prevertebral. See Ganglia, collateral sym-
pathetic,
structure of, 341
of sympathetic trunk, 335
terminal, 335
vertebral. See Ganglia of sympathetic
trunk.
of trigeminus. See Ganglion, semilunar.
vestibular, 188
Gemmules, 43
Geniculate body. See Body.
ganglion. See Ganglion.
Gennari, line of, 283
Genu of corpus callosum, 243
of internal capsule, 258, 262
internum of facial nerve, 175, 176, 180
Glia-cells. See Cells, neuroglia.
Glial sheath, 86
Globus pallidus, 254, 256, 324
Glomeruli, cerebellar, 208
olfactory, 276
of sensory axons, 63
of sympathetic ganglia, 341
Golgi cells of Type II, 44, 87
method of, 361
Goll, column or tract of. See Fasciculus gra-
cilis.
Gowers, bundle of. See Fasciculus anterolater-
alis superficialis.
Granular layer of cerebellum, 208
Gudden, commissure of, 227
Gustatory apparatus, 181
Gyrus (or gyri), angular, 236
annectent, 234
anterior central, 235, 290
ascending parietal. See Gyrus, posterior cen-
tral.
breves or short gyri of insula, 237
callosal. See Gyrus cinguli.
centralis, anterior, 235, 290
posterior, 236, 292
cinguli, 240
dentatus. See Fascia dentata.
diagonal, of rhinencephalon, 267
INDEX 387
Gyrus fornicatus, 240
frontal, ascending. See Gyrus, anterior cen-
tral, ^r
inferior, 235
middle, 235
superior, 235, 240
fusiform, 240
hippocampal, 116, 240, 277
insulae, 237
limbic. See Lobe, limbic.
lingual, 239, 292
longus insulae, 237
marginalis. See Gyrus, superior frontal,
olfactory, lateral, 116, 266, 277
medial, 116, 266
orbital, 241
postcentral. See Gyrus, posterior central,
posterior central, 236, 292
precentral. See Gyrus, anterior central,
rectus, 241
subcallosus (pedunculus corporis callosi), 267
supracallosal, 244, 270
supramarginal, 236
temporal, inferior, 236
middle, 236
superior, 236
transverse, 236, 293
uncinatus. See Gyrus, hippocampal.
HABENULA. See Nucleus habenulae.
Hearing, organs of, 185, 186, 187, 309
Heart, innervation of, 353
Heat, sensations of, 105, 306
Hemianopsia, 228
Hemiplegia, 323
Hemispheres, cerebellar, 197, 198
cerebral. See Cerebral hemispheres.
Hilus nuclei olivaris, 141
Hind-brain. See Metencephalon and Rhomben-
cephalon.
Hippocampal gyrus, 116, 240, 277
commissure, 231, 271, 280, 296
Hippocampus, 250, 269, 277
Histogenesis of cerebellar cortex, 196
of cerebral cortex, 230
of nervous system, 37
of peripheral nervous system, 40
of spinal cord, 38, 39, 42
ganglia, 38, 40
Horizontal cells of Cajal, 285
Horn of lateral ventricle, 246. (See also
Column.}
Hypophysis, 222
in the dogfish, 29
Hypothalamus, 35, 222
in the dogfish, 29
pars mamillaris, 222
optica, 35
INCISURA. See Notch.
Indusium griseum, 244, 270
Infundibulum, 222
Insula, 229, 237
Inter-brain. See Diencephalon.
Interoceptor, interoceptive, 66, 101
Interpeduncular fossa (or space), 115
Interventricular foramen, 26, 118
Intumescentia cervicalis, 73, 84
lumbalis, 74, 84
388 INDEX
Island of Reil. See Insula.
Iter a tertio ad quartum ventriculum. See
Aqueductus cerebri.
JELLY-FISHES, 19
Joints, sensory fibers of, 72
KRAUSE, end-bulb of, 68
LAMINA affixa, 215
alar. See Plate, alar.
basal. See Plate, basal.
medullaris involuta. See Stratum lacunosum.
quadrigemina, 130, 158
rostralis, 223, 243
septi pellucidi, 272
terminalis, 25, 33, 223, 231
Laminae medullares of lentiform nucleus, 254
thalami, 216
Lancisi, nerve of. See Stria longitudinalis me-
dialis.
Lateral line organs, 356
Layers of cerebellar cortex, 208
of cerebral cortex, 286, 287
ependymal, 37
mantle, 37, 42, 196
marginal, 37, 42, 196
of retina, 225
Lemniscus, lateral, 130, 157, 163, 165, 166, 186,
187, 309
medial, 135, 138, 145, 153, 163, 219, 313
spinal. See Tract, spinothalamic.
trigeminal. See Path, secondary afferent, of
trigeminal nerve.
Ligamentum denticulatum, 74
Limen insulae, 237, 268
Line (or lines) of Baillarger, 283
of.Gennari, 283
Linea splendens, 74
Lingula of cerebellum, 197
Lissauer, tract of. See Fasciculus dorsolater-
alis.
Lobe (lobus or lobes) of cerebellum, 197, 198,
200, 201, 202
of cerebrum, 234
frontal, 234
inferior, 28
insular. See Insula.
limbic. See Cyrus fornicatus.
lineae lateralis, 27
occipital, 236, 238
olfactory, 267
optic, 27, 28, 165
parietal, 236
pyriform. See Area, pyriform.
temporal, 235
visceral, 27
Lobule (or lobulus) ansiformis, 201
bi venter, 199
centralis, 197
paracentral, 240, 290
paramedianus, 201
parietal, inferior, 236
superior, 236
postcentral. See Gyrus longus insulae.
precentral. See Gyri breves insulae.
quadrangularis, 198
quadrate. See Precuneus.
semilunaris, inferior, 198
Lobule semilunaris, superior, 198
simplex, 200
Localization of function in cerebellum, 202
in cerebral cortex, 290
in thalamus, 219
Locus caeruleus, 128
Luschka, foramen of. See Aperture, lateral, of
fourth ventricle.
Luys, nucleus of. See Nucleus hypothalamicus.
Lyra. See Commissure, hippocampal.
MACROSMATIC mammals, 265
Magendie, foramen of. See Aperture, medial, of
fourth ventricle.
Mammillary body, 222, 280
Mantle. See Cerebral cortex.
layer. See Layer.
Marchi stain for degenerated nerves, 360
Martinotti, cells of, 285
Massa intermedia, 216
Matter, central gray, 136, 158
gray, 42, 79, 87
white, 42, 79, 86
Medulla oblongata, 114, 118
closed portion of, 119
development, 35. (See also Myelencepha-
lon.)
in the dogfish, 26, 27, 28
fissure, anterior median, 119
posterior median, 119
form, 118, 119, 120, 121, 122
gray matter, 136
internal structure, 132
length, 118
motor nuclei, 170, 174
open portion of, 119
sensory nuclei, 180, 182
sulci, 119
spinalis. See Spinal cord.
Meissner, corpuscles of, 68
plexus of, 351
Meninges, 73, 74
Merkel, corpuscle of, 68
Mesencephalon, 129, 158
development, 24, 31, 35, 36
in the dogfish, 27, 28
form, 129
internal structure, 158
Metamerism, 58. (See also Segmentation.)
Metathalamus, 220
Metencephalon, 31, 33, 36
Meynert, fasciculus retroflexus of, 220
fountain decussation of. See Decussation,
dorsal tegmental.
Microsmatic mammals, 265
Mid-brain. See Mesencephalon.
Mitochondria, 49
Molecular layer of cerebellum, 208
of cerebral cortex, 286
Monakow, bundle of. See Tract, rubrospinal.
Monro, foramen of. See Foramen, interventric-
ular.
Monticulus, 198
Moss-fibers of cerebellum, 209, 210
Motor apparatus, 316
area of cerebral cortex, 290, 317, 318
end-plate, 62
Muscle, branchial, 174
cardiac, innervation of, 353
INDEX
389
Muscle of eyeball, innervation of, 352
of facial expression, innervation of, 192
of larynx, innervation of, 194
of mastication, innervation of, 192
nerve endings in, 62, 72
sense (proprioceptive), 72, 99, 100, 311
skeletal. See Muscle, branchial and somatic.
smooth or unstriated. See Muscle, visceral.
somatic, innervation of, 62, 170
striated. See Muscle, branchial and somatic.
of tongue, innervation of, 194
visceral, innervation of, 61, 174, 177
Muscle-spindles, 72
Myelencephalon, 31, 32, 33, 36
Myelin, 46
sheath. See Sheath.
Myelination in cerebral cortex, 289
in spinal cord, 112
Myotome, 58, 170
NEOPALLIUM, 116, 232, 242
Neothalamus, 219
Nerve (Nervus), abducens, 123, 154, 173, 192
accessory, 123, 176, 177, 194
acoustic, 123, 185, 192
auditory. See Nerve, acoustic.
cardiac, 348, 349
cerebrospinal, 56
chorda tympani, 192, 352
ciliary, 352
cochlear, 149, 185, 193
components, 61. (See also Nerve-fibers.)
cranial, 56, 132, 133, 168
facial, 123, 153, 175, 192
glossopharyngeal, 123, 193
hypoglossal, 123, 173, 194
intermedius, 123, 192
of Lancisi. See Stria longitudinalis medialis.
lingual, 192
oculomotor, 130, 164, 171, 172, 191
olfactory, 191, 265
optic, 191, 225
phrenic, 59
pneumogastric. See Nerve, vagus.
spinal, 56, 58, 65
development of, 40
splanchnic, 348
sympathetic, 345
terminalis, 27, 190
thoracic, 58
trigeminal, 124, 154, 174, 182, 191
trochlear, 125, 163, 173, 191
vagus, 123, 178, 193
vestibular, 149, 185, 193, 314
of Wrisberg. See Nervus intermedius.
Nerve-cells, 43. (See also Neurons and Cells.)
autonomic. See Neurons, sympathetic.
motor, for involuntary muscles, 177
for voluntary muscles, 777
processes, 43
shape, 43
structure, 47
types of, 43, 44
Nerve-endings, encapsulated, 68
free in epidermis, 67
in free arborizations, 67
in hair- follicles, 70, 71
in Meissner's corpuscles, 68
on Merkel's touch-cells, 68
Nerve-endings in muscle-spindles, 71, 72
in Pacinian corpuscles, 69
peripheral, 66-72
plexuses of sensory nerve-fibers, 67
in synapses. See Synapse.
in tactile corpuscles, 68
in tendons, 72
in voluntary muscles, 62
Nerve-fibers, 45. (See also Fibers.)
afferent, 58, 63. (See also Nerve-fibers, so-
matic and visceral afferent.)
autonomic. See Nerve-fibers, preganglionic
and postganglionic.
of cerebellar cortex, 209
of cerebral cortex, 283
classification of, 60
collateral, 43, 97
degeneration of, 52, 105, 106, 107
development, 40, 41
of dorsal root, 95
efferent, 58
exteroceptive, 66
gray. See Nerve-fibers, postganglionic.
interoceptive, 66
to involuntary muscles, 61
medullated. See Nerve-fibers, myelinated.
motor, 59, 62, 94
myelinated, 45, 46, 47, 63, 66, 67, 87
non-medullated. See Nerve-fibers, unmyelin-
ated.
postganglionic, 337, 343
preganglionic, 337, 344
primary motor, 62, 90
proprioceptive, 66, 72
regeneration, 52
of Remak. See Nerve-fibers, unmyelinated.
somatic afferent, 61, 66
general, 168, 182, 192, 193
special, 168, 191, 193
efferent, 61, 62, 168, 191, 192, 194
sympathetic. See Nerve-fibers, postgangli-
onic.
unmyelinated, 47, 63, 66, 67, 87, 98, 104
visceral afferent, 61
general, 168, 181, 193, 335
special, 168, 180, 192, 193
efferent, 61
general, 168, 178, 192, 193, 194, 336
special, 168, 174, 192, 193, 194
to voluntary muscles. See Nerve-fibers, so-
matic efferent and special visceral efferent,
of white rami, 61, 347
substance of brain and cord, 47
Nerve-root. See Root.
Nervous system, autonomic, 339
cranial, 339
craniosacral, 340
sacral, 339
thoracicolumbar, 339, 340
central, 20, 21, 56, 57
cerebrospinal, 58
development of, 24, 32, 36, 37
diffuse, 18, 19, 340
invertebrate, 19, 20, 21, 22
peripheral, 56
subdivisions of, 56
sympathetic, 56, 57, 334
vertebrate, 21, 22
Net, nervous, 19, 340. (See also Plexus.)
39
INDEX
Neural crest, 37
groove, 24, 31
tube, 24, 31, 36
Neurilemma, 41, 46, 47
Neurobiotaxis, 179
Neuroblasts, 37, 39
Neurofibrils, 48, 49, 50
Neuroglia, 85, 86
Neuromuscular end-organ, 72
mechanism, 17
Neuron or neurons, 43. (See also Nerve-cells.}
basket cell, 50
bipolar, 39, 44, 63
chains, 43, 49, 53, 54
concept, 52
of cerebellar cortex, 207, 208, 209
of cerebral cortex, 285
development of, 37
form of, 42
horizontal, of Cajal, 285
interrelation of, 49
lower motor, 318
of Martinotti, 285
motor, 22, 44, 46, 177
multipolar, 44
of olfactory bulb, 275
polarization of, 50
postganglionic, 337
preganglionic, 337, 339, 341
of Purkinje, 207
pyramidal, 43, 44, 285
of retina, 225, 226
sensory, 22, 23, 37, 63
stellate, 285
structure of, 47
sympathetic, 341
theory of. See Neuron concept.
type I, 44, 87
type II, 44, 45, 87, 88
unipolar, 39, 44, 63
upper motor, 317
Neuropil, 20, 21
Neuropore, 31
Nissl bodies or granules, 48, 51
Nodes of Ranvier, 47
Nodule of vermis, 198
Non-medullated fibers. See Nerve-fibers, unmye-
linated.
Notch, anterior cerebellar, 197
posterior cerebellar, 197
preoccipital, 234
Nucleated sheath. See Neurilemma.
Nucleus (or nuclei) of abducens N., 154, 173
accessory cuneate, 138
of accessory N., 194
of acoustic N. See Nuclei, cochlear and
vestibular.
ambiguus, 146, 176
amygdaloid, 249, 257
anterior thalami, 217, 218
arcuate, 140, 143
arcuatus thalami, 218
of Bechterew, 152, 189
caudatus, 253
centralis, superior, 157
of thalamus, 218
of cerebellum, 203, 204
cochlear, 123, 149, 185
commissural, 330
Nucleus of corpus mamillare, 222
cuneatus, 122, 134, 137, 139
of Darkschewitsch, 153
of Deiters, 151, 189
dentatus, 203, 206, 211
dorsalis, 90, 100
of dorsal funiculus. See Nucleus gracilis and
Nucleus cuneatus.
dorsal motor, of vagus, 146, 178
thalamic. See Nucleus, anterior thalami.
of Edinger and Westphal, 178
emboliformis, 203
external round, 138
of facial N., motor, 153, 175, 179
of fasciculus cuneatus. See Nucleus cuneatus.
gracilis. See Nucleus gracilis.
solitarius. See Nucleus of tractus solitarius.
fastigii, 204, 211
of fifth nerve. See Nuclei oi trigeminal nerve,
of fourth nerve. See Nucleus of trochlear
nerve,
funiculi cuneati. See Nucleus cuneatus.
gracilis. See Nucleus gracilis.
globosus of cerebellum, 203
of thalamusr, 218
of glossopharyngeal nerve. See Nucleus am-
biguus and Nucleus of tractus solitarius.
of Goll. See Nucleus gracilis.
gracilis, 122, 134, 137
habenulse, 29, 220
of hypoglossal nerve, 145, 173
hypothalamicus (Corpus Luysi), 223
of inferior colliculus, 165
internal round nucleus, 138
interpeduncular, 115, 164
interstitial, 153
of lateral lemniscus, 157, 187
lateral reticular, of medulla oblongata, 143,
145
lateral thalamic, 217, 219
lemnisci lateralis, 157, 187
lenticular, 254
lentiform, 254
of Luys. See Nucleus hypothalamicus.
of medial longitudinal fasciculus, 153
medial thalamic, 217, 218
mesencephalic. See Nucleus of trigeminal N.
motor, of tegmentum (motorius tegmenti),
145, 161
of nerve-cell, 47
of oculomotor N., 164, 171
olivary, 141, 142
accessory, 142
dorsal, 142
medial, 142
inferior, 141
superior, 151, 186
of origin, 180
pontis, 148, 149
radicis descendemtis N. tngemini. See Nu-
cleus of tractus spinalis of N. V.
red, 159, 160
roof, of cerebellum. See Nucleus fastigii.
ruber. See Nucleus, red.
salivatory, 178
of Schwalbe. See Nucleus, medial vestibular.
semilunar, of thalamus, 218
of sixth nerve, 154, 173
somatic afferent, 182, 185
INDEX
391
Nucleus, somatic efferent, 170
of spinal tract X. V, 136, 144, 155, 182
tecti. See Nucleus fastigii.
tegmental, dorsal, 158
ventral, 158
terminal, 180
thalamic, 217, 218
of tractus solitarius, 146, 181, 330
spinalis N. trigemini, 136, 144, 145, 182
of trapezoid body, 186
of trigeminal N., 154, 156
main sensory, 155, 182
mesencephalic, 155, 184
motor, 155, 174
spinal, 136, 144, 155, 182
of trochlear N., 163, 173
of vagus, motor. See Nucleus, dorsal motor,
of vagus and Nucleus ambiguus.
sensory. See Nucleus of tractus solitarius.
ventral thalamic, 218
vestibular, 151, 188
visceral afferent, 180
efferent, 174, 177
OBEX, 129
Olfactory apparatus, 274-282
bulb, 265, 274
cells of nasal mucous membrane, 274
cortex, 277, 278, 279. (See also Archipallium.)
glomeruli, 276
gyri, 116, 266, 277
lobe, 267
nerve, 265, 275
roots. See Gyri, olfactory.
stria;, 266, 277
tract, 265, 277
trigone, 266
tubercle, 268, 282
Olive (oliva, olivary body), 121
accessory, 142
inferior, 141
superior, 151, 186
Opercula, 230, 237
Optic apparatus, 225
chiasma, 223, 226
cup, 32, 33, 225
lobes, 27, 28, 165
nerve, 225, 226
radiation, 227
tectum. See Cotticulus, superior.
tract, 226
vesicle, 225
Organ of Corti,"185, 186
lateral line, 356
spiral, 185, 186
PACINIAN corpuscles, 69
Pain, apparatus of, 68, 103, 105, 306
Palaeothalamus. See Thalamus, old.
Pallium, 25, 32, 33, 229
Paraflocculus, 202
Paralysis, 322, 323
Paraphysis, 31
Parasympathetic system. See Nervous system,
craniosacral autonomic.
Pars anterior lobuli quadrangularis, 198
basilaris pontis, 124, 147
dorsalis pontis, 124, 149
frontalis capsulse internae, 258, 259
Pars intermedia of Wrisberg. See Nervus in-
termedius.
mamillaris hypothalami, 222
occipitalis capsulae internae, 258, 259
optica hypothalami, 35
posterior lobuli quadrangularis, 198
Path (or pathway), afferent cerebellar, 313, 314
spinal, 98, 303
auditory, 186, 309
cerebello-rubro-spinal, 326
cortico-ponto-cerebellar, 149, 325
craniosacral, 352, 353. 354
efferent, 216
for eye, 352
for heart, 353
for stomach, 353
for submaxillary gland, 352
for urinary bladder, 354
exteroceptive, 66, 101, 102, 302
extrapyramidal motor, 324
final common, 94, 311
motor, 109, 216
for cranial nerves, 320
for spinal nerves, 319
for muscular sense. See Path, proprioceptive.
olfactory, 280
for pain, 103, 104, 105, 306
proprioceptive, 72, 99, 100, 311
secondary afferent, from tractus solitarius,
181
of trigeminal N., 163, 183, 185, 307
vestibular, 190
for thermal sensibility, 105, 306
thoracicolumbar, 352, 353, 354
for touch, 101, 102, 303
vestibular, 190
visual, 226, 227, 228, 310
Peduncle (or peduncles), cerebellar, 204, 205,
206, 211
cerebral, 129, 158
of corpus callosum. See Cyrus subcallosus.
of mammillary body, 222
olivary. See Stalk of superior olive,
of pineal body. See Stalk of pineal body.
Perforated space, anterior. See Substantia per-
forata anterior.
Perikaryon, 43
Pes pedunculi. See Basis pedunculi.
Pia mater, 73
Pineal body. 130
Pituitary body. See Hypophysis.
Plate, alar, 34, 42, 194
basal, 34, 42
neural, 24
roof, of prosencephalon, 213
Plexus of Auerbach, 351
brachial, 58
cardiac, 349
celiac, 349
chorioid, lateral, 251
of fourth ventricle, 128
of third ventricle, 223
esophageal, 349
gastric, 349
hypogastric, 351
intercellular, of sympathetic ganglion, 344
lumbosacral, 58
Meissner's, 351
mesenteric, 349
392
INDEX
Plexus, myenteric, 351
pelvic, 351
pericellular, of spinal ganglion, 65
of sympathetic ganglion, 345
pulmonary, 349
solar, 349
submucous, 351
sympathetic, 345, 348
vesical, 354
Polarity of the neuron, 50
Poles of cerebral hemisphere, 232
Pons (Varoli), 114, 123
basilar or ventral part of, 124, 147
dorsal or tegmental part of, 124, 149
form, 123
internal structure, 147
longitudinal fasciculi, 147
nuclei of, 148
taenia of, 148
transverse fibers of, 147
Ponticulus. See T&nia of fourth ventricle.
Portio major N. trigemini, 125
minor N. trigemini, 125
Postganglionic fibers, 337, 343
Precuneus, 240
Preganglionic fibers, 337, 344
Pressure, apparatus of sensibility to, 66
Presubiculum, 277
Processus reticularis. See Reticular formation of
spinal cord.
Projection centers, 290
fibers, 297
Proprioceptor, proprioceptive, 72, 99, 100, 183,
185, 311
Prosencephalon, 24, 25, 31, 36, 113
Protoplasm, 17
Psalterium. See Commissure, hippocampal.
Pulvinar, 214, 217, 227
Purkinje, cells of, 207
Putamen, 254, 255
Pyramid (or pyramis) of cerebellum, 198
of medulla oblongata, 119, 136
of vermis, 198
Pyriform lobe, 116, 268, 277
RADIATION (or radiatio), auditory or acoustic,
261
of corpus callosum, 243, 245
occipitothalamica. See Radiation, optic.
optic, 227, 261, 264
sensory, 264
thalamic, 216, 217, 260, 263
thalamotemporal, 264
Radix descendens (mesencephalica) N. tri-
gemini. See Root, mesencephalic N. V.
N. facialis, 175
Ramus communicans, 335, 346
gray, 335, 347
white, 335, 347
dorsal, 58
ventral, 58
Ranvier, constrictions or nodes of, 47
Receptor, 19, 53, 91
Recess, lateral, of fourth ventricle, 125
lateralis fossae rhomboideae, 125
optic, 223
pineal, 221
suprapineal, 221
Reflex act, 91
Reflex arc, 20, 53, 91, 92, 93, 327
auditory, 331
of brain stem, 328, 329, 330, 331, 332
for coughing and vomiting, 330 .
of medulla oblongata, 328, 329, 330
myenteric, 340
optic, 332
pupillary, 332, 333
respiratory, 330
scratch, 94
of spinal cord, 91, 92, 93, 94, 328
vestibular, 328, 329
visceral, 340
Regeneration of nerve-fibers, 52
Reil, island of. See Insula.
Respiratory apparatus, 330
Restiform body, 122, 143, 205
medial part of, 205
Reticular formation (or substance), 80, 136, 144
Retina, 225
Rhinencephalon, 25, 32, 115, 265
Rhombencephalon, 25, 31, 32, 35, 36, 113
Rhombic lip, 195
Rod and cone cells, 226
Rolando, fissure of. See Sulcus centralis.
substantia gelatinosa of, 80
tubercle of. See Tuberculum cinereum.
Root of abducens nerve, 123
of accessory nerve, 76, 123
of acoustic nerve, 123
anterior spinal. See Root, ventral,
dorsal, 58, 76, 95, 96, 97
of facial nerve, 123
field. See Sensory root field,
of glossopharyngeal nerve, 123
of hypoglossal nerve, 123
mesencephalic, N. V. 155, 156
of oculomotor nerve, 130
posterior, spinal. See Root, dorsal,
spinal, 78
of trigeminal nerve, 124, 125
of trochlear nerve, 191
of vagus nerve, 123
ventral, 58, 76
Rostrum of corpus callosum, 243
Rudiment of hippocampus, 244, 267, 270
SACCULE, 193
Saccus vasculosus, 28, 29
Scarpa, ganglion of. See Ganglion, vestibular.
Schultze, comma-tract of, 97, 107
Schwalbe, vestibular nucleus of. See Nucleus,
medial vestibular.
Schwann, sheath of. See Neurilemma.
Sea-anemones, 17, 19
Segmentation of spinal cord, 74
Semicircular canals, 193
Septomarginal bundle or fasciculus, 97, 107
Sensation (or sensibility) of cold, 105, 306
of hearing, 185, 186, 187, 309
of heat, 105, 306
muscular, 72, 99, 100, 311
of pain, 68, 103, 105, 306
of pressure, 303
of sight, 225, 228
of smell, 265
of taste, 181
of touch, 66, 77, 101, 303
visceral, 336
INDEX
393
Sensory root field, 59, 60
Septum pellucidum, 243, 272
posterior intermediate, 83
median, 83
posticum, 74
Shark. See Dogfish.
Sheath, glial, 86
medullary. See Sheath, myelin.
myelin, 41, 46, 47
of Schwann. See Neurilemma.
Sight, organs of, 225-228
Smell, organs of, 265-282
Solitary bundle. See Tractus solitarius.
Somesthetic area, 292
Speech, apparatus of, 295, 296
Spider-cells, 86
Spinal cord, 56, 72, 75
cervical enlargement, 73, 79, 84
characters of different regions, 83
columns of gray matter, 79
of white matter. See Funiculus.
of cells. See Cell-columns.
commissures, 80
coverings, 73
cornua. See Columns.
degenerations from brain lesions, 105, 106
from cord lesions, 105, 106
from section of dorsal roots, 106
development, 41, 42
in fetus and infant, 77
fissure, anterior median, 76
funiculi, 82
glial sheath, 86
gray matter or substance, 78, 79, 80, 81, 87
cell-columns, 89, 90
columns, 79
horns. See Columns.
microscopic structure, 87
nuclei. See Cell-columns.
relation to size of nerves, 84
horn. See Column.
internal structure, 85
lumbar enlargement, 74, 81, 84
microscopic structure, 85
relation to vertebral canal, 77
reflex mechanism of, 91, 92, 93
sacral region, 74, 81, 84
segmentation, 74
sulcus, anterolateral, 76
posterior, 76
intermediate, 76
posterolateral, 76
thoracic region, 80, 84
tracts, 95-112, 110
white matter (or substance), 81, 86
area in different regions, 82
microscopic structure, 86, 87
ganglion. See Ganglion.
nerve. See Nerve.
Spiracle, 356
Splanchnic nerves, 348
Splenium corporis callosi, 244
Spongioblasts, 37
Stalk, optic, 32, 225
of pineal body, 221
of superior olive, 151, 175
Stomach, innervation of, 353
Stratum griseum centrale, 163
of superior colliculus, 167
Stratum lacunosum, 278
lemnisci, 167
lucidum, 279
opticum, 167
oriens, 279
profundum, 166, 167
radiatum, 279
zonale of superior colliculus, 167
of thalamus, 216
Stria (or striae) acustica. See Stria medullares
acustica.
of Baillarger, 283
of Gennari, 283
longitudinalis lateralis, 245, 270
medialis, 245, 270
medullaris acustica, 123, 127, 186
thalami, 215, 220, 281
olfactoria lateralis, 266, 277
medialis, 266
semicircularis. See Stria terminalis.
terminalis, 214, 281
Stripe of Baillarger, 283
of Gennari, 283
Subarachnoid space, 73
Subiculum, 277, 280
Substantia alba, 42, 79, 86
ferruginea, 128
grisea, 42, 79, 87
centralis, 136, 158, 163
gelatinosa, Rolandi, 80
centralis, 86
externa. See Sheath, glial.
nigra, 129, 158, 164
perforata, anterior, 267, 282
posterior, 115
reticularis. See Reticular formation,
alba, 144
grisea, 145
Subthalamic tegmental region. See Subthala-
mus.
Subthalamus, 222
Sulcus (or sulci), anterior lateral, 76, 119
parol factory, 239
basilar, 124
callosal. See Sulcus of corpus callosum.
central, of Rolandi, 233
cerebellar, 199
cerebral, 233, 235, 236, 239
cinguli, 239
circularis insulae, 237
of corpus callosum, 239
cruciate, 114
frontal, inferior, 235
middle, 235
superior, 235
horizontalis cerebelli, 197
hypothalamicus, 223
insulae, 237
intermedius, posterior, 76, 127
intra parietal, 236
lateral, of mesencephalon, 130
lateralis, anterior. See Sulcus, anterior lateral.
posterior. See Sulcus, posterior lateral,
limitans, 34, 42, 129
insulae. See Sulcus circularis insulae.
lunatus, 237
median us posterior of spinal cord, 76
of medulla oblongata, 119
occipitalis transversus, 236
394
INDEX
Sulcus of oculomotor nerve, 130
olfactory, 241
orbital, 241
paracentral, 239
parolfactorius, anterior, 239
posterior, 267, 239
postcentral, inferior, 236
superior, 236
postclivalis, 197
posterior lateral, 76, 119
parolfactory, 239, 267
precentral, 235
inferior, 235
superior, 235
prepyramidal, 202
primarius. See Fissura prima.
rhinalis. See Fissure, rhinal.
of spinal cord, 76
subparietal, 239
temporal, inferior, 236
middle, 236
superior, 236
uvulo-nodularis, 203
Sylvius, aqueduct of, 26, 158
fissure of, 233
Sympathetic ganglia. See Ganglion.
system, 50, 57, 334
Synapse, 49, 50, 51, 55
Syncytium, 38
System. See Nervous system.
TACTILE corpuscles, 68
Taenia chorioidea, 214
of fourth ventricle, 126
pontis. See Fila lateralia pontis.
tecti. See Stria longitudinalis lateralis.
thalami, 214, 224
ventriculi quarti, 126
Tapetum, 245
Taste, apparatus of, 181
Tectum mesencephali, 28, 165
Tegmentum, 129, 158
Tela chorioidea of fourth ventricle, 128
of third ventricle, 215, 224
Telencephalon, 36
development, 25, 31, 32, 33
in the dogfish, 27, 28
medium, 212, 229
Temperature, apparatus of, 105, 306
Tendon, nerve endings in, 72
Tentorium cerebelli, 113
Thalamencephalon. See Diencephalon.
Thalamus, 213
development, 35, 213
in the dogfish, 29
ending of sensory tracts in, 219
lamina, external medullary, 216
internal medullary, 216
new, 219
nuclei, 217
old, 218
pulvinar, 218
radiation of, 216, 217, 260, 263
stalks, 263
stratum zonale, 216
thalamocortical fibers, 263
tubercle, anterior, 213
Tigroid bodies. See Nissl bodies.
Tonsil (tonsilla cerebelli), 199
Touch, apparatus of, 66, 71, 101, 303
Tract or tracts, 95. (See also Bundle and Fas-
ciculus.)
bulbospinal, 111
of Burdach. See Fasciculus cuneatus.
central sensory. See Path.
cerebellobulbar. See Tract, fastigiobulbar.
cerebellotegmental, 211, 212
comma, 97, 107
corticobulbar; 165, 260, 321
corticopontine, 147, 164. (See also Tracts,
frpntopontine and temporopontine.)
corticorubral, 161, 260
corticospinal, 109, 133, 136, 147, 165, 260, 320
lateral, 109, 134, 136
ventral, 134, 136
corticothalamic, 263
direct cerebellar. See Tract, dorsal spinocere-
bellar.
dorsal spinocerebellar, 110, 143, 144, 145, 205
efferent, from cerebellum, 211
from cerebral hemisphere, 297
from mesencephalon. See Tracts, tecto-
spinal, tectobulbar, and rubrospinal.
fastigiobulbar, 212
of Flechsig. See Tract, dorsal spinocerebellar.
frontal olfactory projection, 281
frontopontine, 164, 259
of Goll. See Fasciculus gracilis.
of Gowers. See Tract, ventral spinocerebellar.
habenulo-peduncular. See Fasciculus retro-
flexus.
of Helweg. See Tract, bulbospinal.
lateralis minor. See Fasciculus lateralis minor,
of Lissauer. See Fasciculus dorsolateralis.
mamillotegmental, 222, 281
mamillothalamic, 217, 222
mesencephalic, of N. V. See Root, mesen-
cephalic, N. V.
of Meynert. See Fasciculus retroflexus.
of Monakow. See Tract, rubrospinal.
nucleocerebellar, 205
olfactory, 265, 277
olivocerebellar. See Fibers, olivocerebellar.
olivospinal. See Tract, bulbospinal.
optic, 226
pontocerebellar. See Brachium pontis.
pontospinal. See Tract, reticulospinal.
predorsal. See Tract, tectospinal.
prepyramidal. See Tract, rubrospinal.
projection, 297
pyramidal, 109
aberrant, 321
direct, 109
crossed, 109
uncrossed lateral, 320
reticulospinal, 160
rubroreticular, 160, 161
rubrospinal, of Monakow, 110, 145, 161
of Schultze, 107
septomarginal, 97, 107
solitariospinalis, 330
solitary (solitarius), 132, 181, 330
spinal, of N. V., 132, 136, 144
of spinal cord, 94-112, 110
spinocerebellar, dorsal, 100, 314
ventral, 100, 313
spino-olivary, 105
spinotectal, 105, 145
INDEX
395
Tract, spinothalamic, 145, 163, 219, 307
lateral, 102
ventral, 101, 305
strionigral, 164, 263
sulcomarginal, 108
tectobulbar, 161, 167
tectocerebellar, 206
tectospinal, 111, 145, 161, 167
tegmentospinal. See Tract, reticulospinal.
temporopontine, 164, 261
thalamocortical, 263
thalamo-olivary, 145, 219
thalamospinal, 219
transverse peduncular, 369
trigeminothalamic, 183, 185
ventral spinocerebellar, 100, 144, 145, 157, 206
vestibulocerebellar, 190, 206
vestibulospinal, 111, 190, 329
of Vicq d'Azyr. See Tract, mamillothalamic.
Trapezium. See Trapezoid body.
Trapezoid body, 121, 150, 186
Triangle of Gombault and Philippe, 107
Trigone (or trigonum) acustici. See Area
acustica.
collateral, 248
habenulae, 220
hypoglossi, 127
interpedunculare. See Fossa interpeduncula-
ris.
olfactory, 266
vagi. See Ala cinerea.
Trophic unity of neuron, 51
Truncus corporis callosi, 244
Trunk, sympathetic, 335, 346, 347, 348
Tuber vermis, 198, 201
Tubercle (or tuberculum) acusticum. See Nu-
cleus, dorsal cochlear.
anterior, of thalamus, 213
cinereum, 122, 280
cuneate, 121, 137
olfactorium, 268, 282
of Rolando. See Tuberculum cinereum.
Tufted cells, 276
Tiirck's bundle. See Tract, ventral cortico-
spinal.
UNCUS, 240, 269, 277
Utricle, 193
Uvula vermis, 198
VALLECULA of cerebellum, 197
Valve of Vieussens. See Velum, anterior med-
ullary.
Velum, anterior medullary, 125, 128, 155
anticum. See Velum, anterior medullary,
interpositum. See Tela chorioidea of third
ventricle,
medullare, anterius, 125
inferius. See Velum medullare, poste-
rius.
posterius, 202
superius. See Velum, anterior medullary,
transversum, 29, 31
Vena terminalis, 214
Ventricle (or ventricles) of the brain, 25, 26, 27,
117
development, 26, 33, 34
in the dogfish, 27, 28, 30, 31
fourth, 26, 118, 125, 126, 127, 128
lateral, 26, 118, 246
third, 26, 118, 223
Ventriculus lateralis, 26, 246
terminalis, 81
tertius. See Ventricle, third.
Vermis, inferior, 197, 198
superior, 197
Vesicles, cerebral, primary, 24, 25
optic, 225
Vestibular apparatus, 188, 189, 190
Vicq d'Azyr, bundle of. See Tract, mamillo-
thalamic.
Vieussens, valve of. See Velum, anterior med-
ullary.
Visceral innervation, 335
Visual apparatus, 225
receptive center, 292
Visuo-psychic area, 293, 294
Vomiting, mechanism of, 331
WALLERIAN degeneration, 105, 106, 107
Weight of brain, 301
Worms, nervous system of, 19, 20, 21, 22
Wrisberg, nerve of. See Nervus intermedius.
ZONE, cortical. See Center, cortical,
ependymal, 37
mantle, 37, 42, 196
marginal, 37, 42, 196
